From Chapter V
OF MICROBES AND MEN

The family of streptococcal bacteria is responsible for a wide range of human disease—from strep throat to scarlet fever, bacterial pneumonia, and rheumatic fever. Many types of streptococcal bacteria exhibit a phenomenon called molecular mimicry in which they display characteristics of human cells in order to trick the immune system. The cells these bacteria mimic include cells found in the heart, the joints, and even the brain. When you have a bacterial infection, your immune system produces antibodies to attack the invaders. When the invaders are partially disguised through molecular mimicry, they can cause an autoimmune disorder. The immune system recognizes the threat posed by bacterial invaders, but the antibodies it produces attack all the cells that resemble the bacteria—including the body's own cells. That's how some children who have rheumatic fever end up with heart problems—antibodies attack the heart valve because the infecting bacteria resembles it in some ways.

Dr. Susan Swedo, a researcher at the National Institute of Mental Health, believes that certain strep infections can trigger an autoimmune disorder that leads to an antibody-led attack on the basal ganglia, the part of the brain believed to control movement. Researchers call this condition PANDAS—pediatric autoimmune neuropsychiatric disorder associated with streptococcal infection. Parents of children with PANDAS describe heartbreaking transformations, often overnight. Shortly after infection, children suddenly display repetitive tics and uncontrolled touching, as well as serious anxiety.

It's not clear that this is actual host manipulation—that depends on whether the change in behavior helps the bacteria to spread. Theoretically, of course, it's not hard to imagine how uncontrolled, repetitive touching of toys, furniture, and other kids would help the virus to spread. It's also possible that there is a relationship between obsessive-compulsive disorder and strep infections that isn't host manipulation itself, but the by-product of the bacteria's effort to fool the immune system.

One thing is clear—we are just beginning to understand the myriad ways our behavior is affected by infectious agents. One very new avenue of research is exploring the striking possibility that sexually transmitted diseases may actually influence sexual behavior. Now, I'm not suggesting that this kind of influence will transform a happily married man into an insatiable cheat. In fact, that wouldn't necessarily be in the virus's (or fungus's or bacteria's) interest. Too much promiscuity on the part of the host could disable it with other, potentially more damaging, diseases. And that would leave the parasite stuck in a host that couldn't get around. From the sexually transmitted parasite’s point of view, it may want you to have more sex—but not too much sex.

As far as diseases influencing human sexual behavior, some researchers are examining the possibility that genital herpes may affect human sexual feeling in a way that could influence behavior. Two researchers at the Department of Anatomy and Neurobiology at the University of California at Irvine, Carolyn G. Hatalski and W. Ian Lipkin, have speculated that the herpes virus may heighten sexual feeling because it is so intertwined with the nerves that carry those feelings. They wrote:

It is intriguing to speculate that the ganglion infection may modulate sensory input to sex organs leading to increased sexual activity and enhanced probability of virus transmission.

In other words, sometimes the herpes virus may want you to get some action.

The foregoing is excerpted from Survival of the Sickest by Sharon Moalem. All rights reserved. No part of this book may be used or reproduced without written permission from HarperCollins Publishers, 10 East 53rd Street, New York, NY 10022

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