Mad cow disease, AIDS, ebola: all these diseases are apparently new, unknown until recently. Yet they do not come from nowhere. Disease organisms do switch their hosts from time to time; sometimes when they do so, they can have a severe effect. For example, simian hemorrhagic fever virus is a disease of CHECK. Macaques do not usually get it, even if they live with baboons. But if the disease is transferred accidentally--through a dirty hypodermic needle, say--macaques get extremely sick, transmitting the disease to each other by coughing and sneezing. Yet in baboons, the disease has no detectable effect. Thus, predicting which diseases will be able to cross into new species, or knowing what sort of effect they may have, is extremely difficult. All that is certain is that it will happen again.
