What kinds of pathogens cause the damaging chronic diseases we don’t know about? What should we look for? Persistent infections are more likely to cause chronic diseases than short-lived infections because the more time a pathogen has to gum up the body’s machinery, the more likely it is that it will. Internal pathogens are more likely to cause problems than external ones because internal tissues are more delicate and more central to life processes; moreover, internal pathogens must cope with the immune system both to survive and to get out—and this ups the ante. Their persistence in the host provokes immunological destruction of infected cells and inevitable casualties from friendly fire. The tricks that allow a pathogen to avoid immunological destruction can push our immunological systems to the point of self-destruction.     These criteria fit those pathogens that have been traditionally classified as sexually transmitted. They also fit pathogens that are transmitted by less hard-core sexual contact, such as the Epstein-Barr virus, which causes infectious mononucleosis and is transmitted by intimate kissing. Sexually transmitted diseases make up only a small fraction of human diseases, but they are major players in the known chronic diseases caused by infection. Syphilis, infertility due to Chlamydia and gonorrhea, arthritis and ectopic pregnancy caused by Chlamydia, and cervical cancer are just a few of the mentally or physically debilitating chronic diseases caused by sexually transmitted pathogens. Sexually transmitted pathogens are likely to cause a disproportionately large number of the infectious chronic diseases yet to be discovered. We already know of some suspects. Human parvovirus B19, for example, probably contributes to the crises of sickle-cell anemia and to chronic diseases such as arthritis and multiple sclerosis. Some recent evidence indicates that it may be at least partly a sexually transmitted pathogen. In 1999 researchers in the Italian Red Cross reported a comparison of parvovirus B19 infection rates among patients in an STD clinic with rates among healthy blood donors who were used as controls. Almost 40 percent of the clinic patients were infected with B19, compared with only 10 percent of the blood donors.     The other infectious causes of chronic diseases will probably be an eclectic collection much like those that have already been discovered—a grab bag of pathogens. Some of these pathogens may rely on their persistence for transmission, like H. pylori and the papillomaviruses. Others may just by chance have characteristics that allow them to persist in the body in places unrelated to transmission; this category is illustrated by chronic ear infections that are caused by pathogens of the respiratory tract, such as Haemophilus influenzae and Streptococcus pneumoniae. Still other pathogens may cause chronic disease because of long-term disturbances of the body’s machinery that persist even after the pathogen is gone. Some autoimmune responses might fall into this category, such as chronic heart disease caused by streptococcal infections.*38\225\2*

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