As late as 1955 came a victory over poliomyelitis, thanks to a vaccine prepared by the American microbiologist Jonas Edward Salk (1914–[1995]). And yet victory is not total. Right now, the once ravaging disease of smallpox seems to be wiped out. Not one case exists, as far as we know, in the entire world. There are however infectious dis- eases such as a few found in Africa that are very contagious, virtual- ly 100 percent fatal, and for which no cure exists. Careful hygienic measures have made it possible for such diseases to be studied with- out their spreading, and no doubt effective countermeasures will be worked out. New Disease It would seem, then, that as long as our civilization survives and our medical technology is not shattered there is no longer any danger that infectious disease will produce catastrophe or even anything like the disasters of the Black Death and the Spanish influenza. Yet, old familiar diseases have, within them, the potentiality of arising in new forms. The human body (and all living organisms) have natural defenses against the invasion of foreign organisms. Antibodies are developed in the bloodstream that neutralize toxins or the micro- organisms themselves. White cells in the blood stream physically attack bacteria. Evolutionary processes generally make the fight an even one. Those organisms more efficient at self-protection against microorganisms tend to survive and pass on their efficiency to their offspring. Nevertheless, microorganisms are far smaller even than insects and far more fecund. They evolve much more quickly, with individual microorganisms almost totally unimportant in the scheme of things. Considering the uncounted numbers of microorganisms of any particular species that are continually multiplying by cell fission, large numbers of mutations must be produced just as con- tinually. Every once in a while such a mutation may act to make a particular disease far more infectious and deadly. Furthermore, it may sufficiently alter the chemical nature of the microorganism so that the antibodies which the host organism is capable of manufac-
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