The Infection Connection - how infections might be partly responsible for mental illnesses

Harriet Washington
PSYCHOLOGY HAS LONG HELD THAT MENTAL ILLNESS IS BORN OF ADVERSE EXPERIENCES. MORE RECENTLY, RESEARCH HAS POINTED THE FINGER AT FLAWED GENES. NOW A THIRD CULPRIT MAY BE EMERGING: INVASION BY BACTERIA AND VIRUSES.

Eight-year-old Seth broke from the grasp of Jane, his harries mother, for the third time in 10 minutes. Tering across the emergency room, he stopped short, transfixed by a piece of paper lying on the floor. His ref-rimed eyes seemed to bulge from their sockets and his mouth twitched violently, as if he were in pain. Indifferent to Jane's pleas to stop, he proceeded to pick up from the floor every piece of paper, no matter how filthy, with hands that were reddened and raw. It was the state of his hands that had precipitated the trip to the hospital: Seth had spent most of the night in the bathroom, washing them over and over.

With his head jerking spasmodically and his fingers pecking at pieces of paper and cigarette butts, the boy resembled some strange overgrown bird. Then, suddenly terrified, he flew back to Jane and began pulling on her arm. "Mommy, Mommy, let's leave!" he whimpered. "They're going to kill us. They're coming!"

Jane tried her best to calm him, but she too was beginning to panic. Two days before, Seth had been a perfectly normal little boy whose most serious health problems were the occasional cold or sore throat. He had become mentally ill overnight.

What caused Seth's anxiety, his tics, his obsessive-compulsive behavior? Astonishingly, it was probably that minor sore throat, his doctors concluded. Today, scientists are increasingly coming to recognize that the bacteria and viruses that frequently invade our bodies and cause sore throats and other minor ailments may also unleash a host of major mental and emotional illnesses, including anorexia, schizophrenia and obsessive-compulsive disorder.

It is a theory sharply at odds with earlier views of the genesis of psychological illness. Followers of Freud long held that mental and emotional trouble is primarily the result of poor parenting, especially by mothers. Indeed, until about 30 years ago, psychoanalysts frequently placed the blame for schizophrenia on "schizophrenogenic" mothers. Obsessive-compulsive disorder, also, was put at Mom's door. "It was thought to be the result of harsh toilet training," observes Susan Swedo, M.D., chief of pediatrics and developmental neuropsychiatry at the National Institutes of Mental Health. But such theories, which added immeasurable guilt to the burdens of parents with mentally ill offspring, have turned out to have little evidence to back them up, most experts now agree.

Instead, in recent years, the focus has shifted to genes as the main source of mental illness. Faulty DNA is thought to be at least partly responsible for, among other problems, anxiety and panic disorders, schizophrenia, manic depression and antisocial personality disorder, which is characterized by impulsive, excessively emotional and erratic patterns of interpersonal behavior.

Yet genetics doesn't appear to wholly account for the occurrence of major psychiatric ailments. If heredity alone were to blame, identical twins would develop schizophrenia with a high degree of concordance, but in fact in only 40% of cases in which one identical twin has the disease does the other twin have it as well. Autism, though it has been observed to run in families, also strikes five of every 10,000 children apparently arbitrarily Nor can depression and other affective disorders be completely explained by damaged DNA. Says Ian Lipkin, Ph.D., a neuroscientist and microbiologist at the University of California at Irvine: "Genetics doesn't hold the key to understanding how to fit these square pegs into round holes."

Bacteria and viruses may be that key, but scientists have been slow to grasp the idea. Consider the case of syphilis, which is caused by the bacterium Treponema pallidum. In its final, or tertiary, stage, the disease can precipitate psychiatric problems like dementia, mania, depression, delusions and Tourette's like tics. Though some scientists' suspected a connection between infection with the bacterium and the mental disturbances that may take three to live decades to emerge, the link became widely accepted only in the 1940s after the introduction of the antibiotic penicillin as a treatment for syphilis. In the interim, patients with syphilis who later developed psychiatric problems were often institutionalized as crazy But even with the link established, Freud's theories were in ascendance and few scientists were willing to consider that microbes might be a common source of other mental illness.

Now, decades later, infection has emerged as a prime suspect in psychological illnesses. The inadequacy of genetic and experiential explanations has prompted scientists to look elsewhere--and their gaze has come to rest on physical ailments, such as heart disease, cancers and ulcers, that in some cases have an infectious origin. Could the same be true, they wonder, for mental and emotional ills?

Improved technology has made it easier to find out. Since active only when inside other living creatures, microbes are notoriously hard to grow, and therefore study, in the lab, but scientists' ability to do so has increased steadily over the last few decades. Other tools have allowed researchers to see their quarry more clearly. For about a decade, microbiologists have used a technique called polymerase chain reaction, or PCR, to replicate a small piece of genetic material over and over until it forms a quantity large enough to study--and large enough to show the lingering traces of an infection. A new variant of PCR, called representational difference analysis, introduced in 1994, allows scientists to go one step further and compare the differences between two separate pieces of DNA (including healthy and diseased segments, for instance). And the refinement of electron microscopes has permitted researchers to follow the "footprints" left by infection in patients' cerebrospinal fluid.

The introduction of CAT scans in the early 1970s has been another leap forward. Before then, only an X-ray--or an autopsy--could reveal damage to the brain, and then only in its grossest form. CAT scans show subtle changes that can be tracked over time, giving researchers a more accurate sense of a microbe's impact. MRI, developed in the early 1980s, has added three-dimensionality to pictures of the brain, and PET scans, invented soon after, have added motion. Now bacteria and viruses can be caught red-handed.

Still, teasing out the tie between microbes and psychological problems has proved a difficult task, in no small part because of the cunning and guile of the pathogens themselves, which have many ways of attacking our bodies and brains.

T. pallidum, syphilis' causative bacterium, proceeds in straightforward fashion: it attacks and kills brain cells. After entering the body--usually through sexual intercourse, or through the mother's placenta into her fetus--the bacterium travels along the lymph system until it arrives at the brain. Once it lodges there, it spares few structures, inflaming some neurons and stripping the myelin, or insulation, from others. (Without myelin, nerve impulses are slowed or stopped altogether.)

Other microbes are more devious. The human immunodeficiency virus, for example, which can cause anxiety, delirium, psychosis and suicidal impulses, uses a Trojan horse strategy.. Instead of directly attacking brain cells, it infects macrophages, the immune-system enforcers that roam the bloodstream, engulfing foreign cells that may pose a threat to the body. After hitching a macrophage ride into the brain, HIV cranks out cytokines, protein peptides that kill off neurons. The virus Chlamydia pneumoniae may use a similar trick-slipping into the brain and manufacturing cytokines--to cause Alzheimer's disease.

Streptococcal bacteria, like those that cause strep throat, take yet a different tack. As they invade the body, they automatically trip the body's defense alarm, calling up the immune system's antibody soldiers. But once the skirmish begins, the bacteria camouflage themselves within that very immune system. Like a wolf in sheep's clothing, they disguise themselves through molecular mimicry, cloaking themselves in proteins that imitate the body's own proteins, and thereby elude attack. For reasons scientists don't yet understand, the antibodies of some people--perhaps those with a genetic predisposition--then turn on their bodies' own tissues, assailing neurons in the basal ganglia. These structures, located between the more primitive lower brain and the higher cortical centers, help interpret information from the senses and are key to healthy emotions and behavior.

The self-destruction instigated by streptococcal bacteria can be particularly dangerous to young children. Because their immune systems are "naive," or underdeveloped, "their bodies mount a vigorous response to streptococci, but it is not terribly effective, and their antibodies wind up injuring their own neurons instead of the bacteria," explains Swedo. She believes that such self-induced damage leads some children to develop obsessive-compulsive disorder.

In an experiment conducted earlier this year, Swedo replaced the blood plasma of 28 children who suffered from OCD (and who had elevated levels of streptococcus antibodies) with healthy donor plasma, reasoning that such a switch would remove the trouble-making antibodies. Within a month, the incidence of tics declined by half, and their other OCD symptoms were reduced by 60%. "That was really gratifying," says Swedo. "It means that OCD is a medical illness, and if you catch it before there is scarring in the brain, you can cure it."

OCD isn't the only mental illness associated with streptococcus. Though anorexia nervosa has been tied to a distorted body image, societal pressure to be thin, discomfort with developing sexuality, and other emotional and cultural factors, doctors have also noted that the eating disorder sometimes appears or worsens after a case of strep throat. Streptococcus has been implicated as well in Tourette's syndrome and in Sydenham's chorea, which makes the arms and legs of those afflicted jerk in a manner often likened to dancing.

Researchers think that the seeds of yet other mental illnesses may be planted while a fetus is still in the womb, when the pregnant mother-to-be becomes infected. Flu epidemics have been followed a generation later by waves of schizophrenia in England, Wales, Denmark, Finland and other countries, and a recent study published in the New England Journal of Medicine reports higher rates of schizophrenia among children born in crowded areas in cold weather--conditions hospitable to respiratory ailments.

Scientists suspect that in such cases a virus, such as the one that causes influenza or a newer candidate, the Borna virus, may insinuate itself into the fetal brain at a crucial stage of development. The microbe then subtly deranges the brain's neural connections in a process that becomes apparent only as the brain reaches full maturity, in early adulthood. In people with schizophrenia, parts of the brain--the cortex, thalamus, limbic system and basal ganglia--shrink, while crevices and fluid-filled spaces enlarge by as much as half, and the brain's chemical balance shifts. Such changes might well be the terrible legacy of a prenatal virus.

Microbes that cause mental illness can also enter the body another way--on one fork. In the mid-1990s, an outbreak of Creutzfeldt-Jakob disease struck fear into meat-eaters, especially in England, where 35 people died after eating infected beef. While alive, the victims of what was dubbed "mad cow disease" exhibited bizarre symptoms such as continual screaming, inappropriate laughter, failure to bathe and compulsive walking.

Scientists theorized that the "mad cows" became infected because they were fed on sheep afflicted with the disease known as scrapie, but a similar infection endemic to cows may be to blame. (Scrapie in sheep, bovine spongiform encephalitis in cows and Creutzfeldt-Jakob disease in humans are all believed to be caused by what's known as a prion, or infectious protein, which acts in a manner similar to a virus.) Contaminated human growth hormone, corneal transplants and surgical instruments have also been suspected of communicating the disease to humans. Earlier this year, the British medical journal The Lancet reported that multiple surgeries and living on a farm are risk factors for CJD. Though the disease is considered rare, Yale neuropathologist Laura Manueldis, M.D., who thinks that the causative agent is actually a small virus rather than a prion, suspects that CJD is more common than we believe, and is often misdiagnosed as Alzheimer's.

Why doesn't every child with a strep throat develop an anxiety disorder? Or dementia strike every adult with syphilis? Our bodies protect us from most invasions by unfriendly microbes, but the vulnerable--those with poor health, weakened immune systems or, perhaps, genetic susceptibility--are less equipped to fend off the viruses and bacteria that may eventually cause mental illness.

Continuing research is likely to bring more effective vaccines and antibiotics, but prevention and treatment will have to be every bit as shrewd as the infectious agents themselves, which spread and reproduce in seemingly countless ways. More worrisome is the fact that illness-causing organisms may be outwitting our antimicrobial strategies. With the development of antibiotics and antivirals, researchers had hoped to vanquish viruses and bacteria once and for all. But they underestimated the wily ingenuity of these microbes, which soon produced strains resistant to the new miracle medicines.

Likewise, scientists at one time believed that infectious agents evolved to become less potent, if only to ensure their own survival: the longer an infected host stays alive, after all, the greater the number of people who can be exposed to the pathogen, and the more likely the microbe is to reproduce itself. But recent research has turned up a less reassuring reality: under some conditions, microbes can flourish with increased virulence in crowded modern cities. These densely populated areas can maintain dangerous pathogens that otherwise might whip through a community and then have nowhere else to go.

To fight these pathogens successfully, doctors will have to tailor the treatments to the specific disease. Sometimes, for example, they may want to relieve an infected patient's fever and inflammation; at other times, fever must be left alone to kill off heat-sensitive viruses. Syphilis can be cured with penicillin, and doctors have had some success in prescribing antibiotics to people with anorexia. Other illnesses require more involved treatment, such as Swedo's plasma replacement for children with OCD, and some ailments cannot be helped much at all. The only way to treat HIV dementia, for example, is to try to slow the replication of the virus itself. No current therapy does this permanently, and many HIV-positive patients still suffer psychiatric symptoms. (In fact, suicide is a leading cause of death among the HIV-infected.) There's no treatment for Creutzfeldt-Jakob disease beyond palliative care, and by the time influenza-induced schizophrenia becomes apparent, the neurological damage is already done.

Still, science continues to offer hope that one day such infectious agents will be controlled or even eliminated. That day can't come too soon for Swedo, a pediatrician by training. "When I practiced medicine at Memorial Hospital in Chicago, I saw parents suffer horribly when they lost their children to leukemia," she says. "When I came to the NIMH, I began to see parents lose their children to OCD and schizophrenia. These parents' grief is so much more profound. The fact that their children's illnesses are socially unacceptable makes their pain almost unbearable." Now that we know many psychiatric ills begin with a microbe, rather than a suffocating mother or remote father, "we can start treating mentally ill people without the shame and blame. We can treat them medically."

WHAT YOU CAN DO

Since we know so little about the viruses and bacteria that cause some types of mental illness, it makes sense to avoid them when you can. Here, advice on preventing infection from leading scientists (much of which sounds uncannily like Mom's):

* Eat a healthy diet.

* Get plenty of sleep.

* Reduce stress.

* Get a flu shot each fall. (I loved this article until this sentence. I guess the writer doesn't know the flu vaccine is full of bacteria and virus!)

* Be aware that sex with multiple partners raises your risk of infection.

* Wash your hands frequently. (Though you might be tempted to use one of the many antibacterial soaps on the market, such cleansers may promote the growth of drug-resistant "superbugs.")

* Don't eat meat that isn't well-cooked, or if you have any questions about how it was stored or prepared.

* Find out whether you have any physical conditions that increase your risk of infection, and discuss preventive steps with your doctor. If, for example, you have a prolapsed mitral valve (a valve in the heart that doesn't close properly, increasing risk of infection), your doctor may recommend that you take antibiotics before having dental work done.

* Pay attention to your body. Even minor symptoms--fever, chills, a sore neck--can mean you're doing battle with an infection, and you should discuss them with your doctor. Pay special attention to your children's symptoms.

* If you are prescribed antibiotics, finish the entire course of therapy.

* If you work in a medical setting--even as a technician or clerk--be sure to seek vaccinations against hepatitis and other pathogens you may acquire from patients.

* If you're pregnant, take special care to avoid respiratory ailments such as influenza. Avoid contact with cat litter, which can harbor Toxoplasma, a microbe that can cause birth defects or even spontaneous abortion.

COPYRIGHT 1999 Sussex Publishers, Inc.
COPYRIGHT 2000 Gale Group



For the complete American Dental Hygienists' Association's (ADHA) article: http://www.adha.org/CE_courses/course8/pica.htm

Nutritional deficiencies of minerals, iron, and zinc appear to be a common theory concerning the etiology of pica. Sayetta reported studies of individuals with pica with low iron and zinc levels whose pica behavior diminished when iron or zinc supplements were given.40 However, empirical evidence remains unconfirmed. Sensory and physiologic theories focus on the finding that individuals with pica claim to enjoy the taste, texture, or smell of the item they are eating. Psychosocial theories related to pica have examined the association of family stress such as maternal deprivation, parental separation, parental neglect, and child abuse involving pica. Investigations have examined an underlying biochemical disorder, like a diminished dopaminergic neurotransmission, but correlations have not been identified empirically.37,38

Others have proposed that pica is part of the obsessive-compulsive disorder (OCD) spectrum of diseases, in which practicing a ritualistic behavior leads to relief of tension and anxiety.41-43 Risk factors for pica include parental/child psychopathology, family disorganization, environmental deprivation, pregnancy, epilepsy, brain damage, mental retardation, and developmental disorders.37

Manifestations of pica vary and include inherent toxicity, obstruction, excessive caloric intake, nutritional deprivation, infection, and injury. Of concern is the toxicity associated with ingestion of lead or other heavy metals. Physical manifestations of lead poisoning can include neurologic (e.g., irritability, lethargy, incoordination, headache, cranial nerve paralysis, seizures, coma, and death) and gastrointestinal (e.g., constipation, abdominal pain, colic, vomiting, anorexia, diarrhea) symptoms. Gastrointestinal symptoms, such as mechanical bowel problems, ulcerations, perforations, and obstructions may occur due to ingesting substances that are undigestible, such as hair eating. Infections and parasitic infestations are also a concern. Toxocariasis is the most common soil-borne parasitic infection associated with pica and can lead to fever, hepatomegaly, malaise, cough, myocarditis, encephalitis, retinal lesions, and loss of vision.

Diagnosis of pica can be difficult and frequently depends on selfreporting. Accurate diagnosis is often hindered by reluctance to report the practice as well as the secretiveness on the part of individuals with pica and their families. In suspected cases, laboratory studies that may be performed include a complete blood count, iron level, ferritin level, lead level, electrolytes, and liver function studies. Abdominal radiographs, upper and lower GI barium examinations, and an upper GI endoscopy may be needed to evaluate for intestinal obstructions, bezoar formation, or parasites. Stool cultures can be used to rule out ova and parasites. Parents should be interviewed about the dietary habits and pica behaviors of children. Furthermore, individuals with pica symptoms may be assessed for OCD and impulse control disorders through psychological evaluations.
 



Diseases of the Mind
Bacteria, viruses and parasites may cause mental illnesses like depression and perhaps even autism and anorexia
By Janet Ginsburg
Newsweek International
Dec. 1 issue - Olga Skipko has had the good fortune to live most of her adult life in the Polish village of Gruszki, in the heart of the Puszcza Bialowieska, one of Europe's most beautiful forests and home to wolves, lynxes and the endangered European bison. Unfortunately, the forest is also a breeding ground for disease-carrying ticks. Skipko, 49, thinks she was bitten about 10 years ago, when she began having the classic symptoms of Lyme borreliosis, a tickborne nervous-system disease: headaches and aching joints. She didn't get treatment until 1998. "I was treated with antibiotics and felt a bit better," she says.

That was only the beginning of her troubles. A few years later, she began to forget things and her speaking grew labored. It got so bad that she had to quit her job in a nursery forest and check herself in to a psychiatric clinic. "I hope they will help me," she says. "I promised my children that when I come back home, I will be able to do my favorite crosswords again." Doctors ran a battery of tests and concluded that her mental problems were the advanced stage of the Lyme disease she had contracted years ago.

Scientists have long known that some diseases can cause behavioral problems. When penicillin was first used to treat syphilis, thousands of cured schizophrenics were released from mental asylums. Now, however, scientists have evidence that infections may play a far bigger role in mental illness than previously thought. They've linked cases of obsessive-compulsive disorder, bipolar disorder and schizophrenia to a variety of infectious agents, and they're investigating autism, Tourette's and anorexia as well. They're beginning to suspect that bad bugs may cause a great many other mental disorders, too. "The irony is that people talked about syphilis as the 'great imitator'," says University of Louisville biologist Paul Ewald, "but it may be the 'great illustrator'-a model for understanding the causes of chronic diseases."

Mental illnesses constitute a large and growing portion of the world's health problems. According to the World Health Organization, depression is one of the most debilitating of diseases, on a par with paraplegia. Psychiatric illnesses make up more than 10 percent of the world's "disease burden" (a measure of how debilitating a disease is), and are expected to increase to 15 percent by 2020. Much of this may be the work of viruses, bacteria and parasites. Psychiatrist E. Fuller Torrey, of the Stanley Medical Research Institute in Maryland, has found from studying historical asylum records that hot spots-higher-than-normal incidences-of mental illness can shift, much like infectious-disease outbreaks, which lends credence to the notion that infectious agents play a big role. "Mental disorders are the major chronic recurrent disorders of youth in all developed countries," says Harvard policy expert Ronald Kessler, who directs the WHO's mental-health surveys.

Perhaps the most well known disease that's been linked to mental disorders is Lyme disease, which is caused by the Borrelia burgdorferi germ. First identified in the mid-1970s among children near Lyme, Connecticut, the disease has long been known to cause nervous-system problems and achy joints if left untreated. Now scientists are finding that Lyme disease can also trigger a whole smorgasbord of psychiatric symptoms, including depression. One New York man (we'll call him Joe) found out firsthand how debilitating the disease can be. When he began having bouts of major depression back in 1992, he had forgotten all about the tick bite he had gotten four years earlier. He spent two years in a blur of antipsychotic drugs, mental institutions, jails and suicide attempts. On a hunch, a doctor at a psychiatric hospital in New Jersey had Joe tested for Lyme disease. After an intensive course of antibiotics, Joe's improvement was dramatic and immediate. "I started to have this fog lift," he recalls. Still, he will probably have to be on psychotropic drugs for the rest of his life.

Some psychiatrists fret that there may be thousands of people suffering from Lyme-induced depression without knowing why. Not only is Lyme disease tricky to diagnose-not everybody gets the circular rash, and lab tests still aren't wholly reliable-it can take a decade or more for mental disorders to set in. The U.S. Centers for Disease Control says that nine out of 10 cases of Lyme diseases remain unreported. There are 15 species of borellias-making them the most common tickborne disease-producing bacteria in the world.

For its part, the parasite Toxoplasma gondii, which can be found in undercooked meat and cat feces, can lead to full-blown psychotic episodes. Some studies suggest that the parasite stimulates the production of a chemical similar to LSD, producing hallucinations and psychosis. Even when the parasite lies dormant in muscle and brain tissue, it can affect attention span and reaction time in otherwise healthy people. Researchers at Charles University in Prague have discovered that people who test positive have slightly slower-than-average reaction times and-possibly as a result-are almost three times as likely to have car accidents. That's a disturbing prospect, considering that the disease is so widespread: billions of people are thought to be infected.

Even a simple sore throat can lead to psychiatric problems. Few children avoid coming down with a streptococcus infection, also known as strep. Scientists now think that one in 1,000 strep sufferers also develops abrupt-onset obsessive-compulsive disorder (OCD) in a matter of weeks. Strep bacteria trigger OCD by igniting an overzealous response from the immune system, which attacks certain types of brain cells, causing inflammation. Symptoms generally die down after a few months but can flare up again, especially if there's another bout of strep, says Susan Swedo, a childhood-disease expert at the National Institutes of Health. The most effective treatment, still experimental, is to filter out the misbehaving antibodies from the blood. Best is to treat strep early on.

The specter of a depression germ or contagious obsessive-compulsive disorder is unnerving, but it also opens up many more treatment options-antibiotics, vaccines, checking for ticks. Geneticists believe that diseases may trigger the onset of inherited mental illnesses by activating key genes. Avoiding and treating infection may be just as important as the genes you inherit, and a whole lot easier to do something about.
 



Abnormal social behaviors in young and adult rats neonatally infected with Borna disease virus.
a.. Lancaster K, Dietz DM, Moran TH, Pletnikov MV.
Division of Neurobiology, Department of Psychiatry and Behavioral Sciences, Johns Hopkins University School of Medicine, 600 North Wolfe Street, CMSC 8-121, Baltimore, MD 21287, USA.

Autism spectrum disorders (ASD) have been the focus of a great deal of research and clinical speculation. This intense interest relates to both the perplexing pathogenesis and devastating consequences of these disorders. One of the obstacles to understanding the pathogenesis of autism and to developing efficient treatment has been the paucity of animal models that could be used for hypotheses-driven mechanistic studies of abnormal brain and behavior development and for the pre-clinical testing novel pharmacological treatments. In this report, we briefly review our animal model of ASD based on neonatal Borna disease virus (BDV) infection and present new data about abnormal social interaction in adult BDV-infected rats. We found that neonatal BDV infection profoundly affected social behaviors in adult rats. Compared to the control rats, both 90- and 180-day-old infected rats spent less time in active social interaction and more time in following their partners. In the intruder-resident test, the BDV-infected resident rats exhibited less aggression towards the intruders and showed more the following-the-intruder behavior. The following-the-partner behavior may be an example of "stereotypic" activity due to BDV-induced abnormal social communication between rats. The previously published results and present findings indicate that neonatal BDV infection significantly altered the normal pattern of social interaction in rats. Co-localization of activated microglia and dying Purkinje cells in BDV-infected rats suggests that the BDV model could be used to study a pathogenic link of Purkinje cell dropout and neuroinflammation to abnormal social behaviors.

PMID: 16860408 [PubMed - as supplied by publisher]

 




http://health.msn.com/centers/cancer/articlepage.aspx?cp-documentid=10015556
1&GT1=9086

Cancers You Can Catch
New evidence shows viruses might cause one of every five cases.
By Korey Capozza for MSN Health & Fitness
Find More

Most people know that viruses can cause a host of unpleasant illnesses skin rashes, stomach flu and the common cold, to name a few. But many of us may not realize these clever microorganisms play a key role in triggering a surprising number of cancers. In fact, one-fifth of cancers may be linked to
viral pathogens. Every year, new discoveries seem to confirm this provocative relationship.

The question of why and how cancer begins to grow in previously healthy tissue has long perplexed scientists. But in the last 25 years, more and more evidence seems to point to a common culprit: Infectious microbes. The revolutionary discovery by Australian scientists that stomach ulcers are likely caused by Helicobactor pylori, or H. pylori, bacteria sparked new interest in the role that bacteria and viruses may play in disease development.

Today, we know that H. pylori also causes stomach cancer, and that infection with two strains of the human papilloma virus (HPV) lead to cervical cancer. In the case of the latter, the viral connection prompted a new cervical cancer vaccine targeted for preteen girls. The potentially contagious cancers list goes on: The hepatitis B virus is related to liver cancer, certain retroviruses are linked to T-cell leukemia, and the Epstein-Barr virus plays a role in some carcinomas. Another virus, named XMRV, made headlines last year when researchers discovered that it may be related to prostate cancer.

"It seems clear that more and more viruses will be implicated in more cancers," says Dr. Robert Ferris, associate professor of immunology at the University of Pittsburgh Cancer Institute. However, certain cancers in particular seem to be associated with parts of the body exposed to the "outside world," such as the mouth, throat and reproductive areas. That's because viruses enter the body through everyday activities like eating, breathing and sexual activity, notes Ferris.

So how do common viruses turn healthy cells into cancerous ones?

Once inside the body, the virus integrates its genetic material with the host cell's DNA. These genetically altered, ³corrupted² cells begin to grow uncontrollably, eventually leading to cancer. In the case of cervical cancer, for example, the HPV virus hijacks cervical cells and turns them into malignant growths.

Not everyone who is infected by these viruses will develop cancer and this is one of the fascinating, if not fear-reducing, mysteries of the virus-cancer connection. For example, while most* women who develop cervical cancer have had a human papilloma virus (HPV) infection, not all women with HPV infection will develop cervical cancer. Studies have shown that 70 percent of new HPV infections resolve on their own within one year, and as many as 91 percent clear within two years. The same holds true for Epstein-Barr virus (EBV) most people are infected with EBV, but very few will develop Hodgkin's lymphoma.

It's clear that the immune system plays a critical role in keeping these high-risk viruses in check. In most cases, the body's defenses clear the infectious agents without any signs or symptoms. That¹s because the immune system is constantly working behind the scenes, patrolling the body for foreign agents and destroying disease-causing microbes before they can tamper with the DNA of healthy cells. In some cases, though, the immune system fails to perform these tasks and the virus seizes the opportunity. Old age, underlying disease or unhealthy habits may weaken the body's defenses, giving these viruses a leg up. "The immune system, like most parts of the body, gets rickety with age and less able to prevent cancer," Ferris says.

The key to preventing some of these cancers simply may be keeping the immune system working in tip-top shape. Eating well, exercising and avoiding
alcohol and cigarettes are good ways to keep your body¹s natural defenses functioning at their best. "You don't want to give the virus an advantage by
smoking or drinking,² says Ferris. ³These habits damage cells and give viruses an entrée into the weakened cells." Viruses are everywhere and are contagious, but there is one significant upside to the virus-cancer link: we have tools which can prevent infection in the first place. For example, the new cervical cancer vaccine, a phenomenal breakthrough, can prevent women from becoming infected with the HPV vaccine and thereby eliminate the risk of cervical cancer.

Interestingly, the HPV vaccine may prevent some mouth, throat and larynx (voice box) cancers, which also appear to be linked to the HPV virus. The
hepatitis B vaccine is another weapon in the fight against cancer. It effectively protects against the liver cancer-associated virus. And because these two viruses often are sexually transmitted, use of condoms during intercourse can prevent exposure to these disease-promoting bugs.

As the old adage goes, an ounce of prevention is worth a pound of cure. Or as Allen Lichter, CEO of the American Society for Clinical Oncology, recently said: "The easiest cancer to cure is the cancer we don't get because we prevented it." Ironically, viruses may both cause and kill cancer. Viruses have evolved over millions of years to selectively target certain cells while leaving other tissue unharmed. For this reason they are, in some ways, the ideal anti-cancer weapon.

Researchers have shown that viruses, such as HIV and inactivated measles, can be used to carry a therapeutic gene to the precise location of the cancer. For example, studies have shown that a benign altered virus can trick the immune system into attacking pancreatic cancer cells, and the measles virus may effectively identify and neutralize ovarian cancer cell lines. This cutting edge of cancer research holds huge promise for the development of effective, non-toxic cancer treatments of the future. Puts viruses in a whole new and positive light, doesn¹t it?

Correction, 2/23/2007: This article originally stated that "all women who develop cervical cancer have had a human papilloma virus (HPV) infection." According to the U.S. National Institutes of Health, HPV infection is the primary risk factor for cervical cancer, but other risk factors include a high number of full-term pregnancies, use of oral contraceptives, and smoking. This error has been corrected. Return to the corrected sentence.

Korey Capozza is a San Francisco-based writer and regular contributor to MSN Health & Fitness.