diseases has been hard to demonstrate, and there appear to be relatively few documented examples in nature. Influenza is probably the best example of a virus for which the importance of new variants (i.e., antigenic drift) can clearly be shown. Variants of the hepatitis B virus also have been shown recently to cause disease. However, cases like these are greatly outnumbered by instances of new diseases or outbreaks resulting from microbial traffic between species. Cross-species transfer of infectious agents is often the result of human activities.

The evolution of viruses is constrained by their requirement for being maintained in a host. It would therefore seem that new variants of nonviral pathogens, such as bacteria, would be more common than new forms of viral pathogens since nonviral organisms are less constrained by host requirements. However, most nonviral pathogens usually show a clonal origin (Selander and Musser, 1990; Musser et al., 1991; Tibayrenc et al., 1991a,b). That is, they appear to be derived from a single ancestor, suggesting that the evolution of a successful new pathogen is a relatively rare event. When it does occur, the new microbe probably originates in a single geographic area and is disseminated through channels of microbial traffic. One implication of this model is that the control of ''new" diseases may be more likely if the new variant is identified early (e.g., by worldwide infectious disease surveillance) and steps are taken to prevent its further dissemination.

It is likely that emerging pathogens generally are not newly evolved. Rather, it appears that they already exist in nature. Some may have existed in isolated human populations for some time; others, including many of the most novel, are well established in animals. Infections in animals that are transmissable to humans are termed zoonoses. As discussed in Chapter 1, throughout history rodents have been particularly important natural reservoirs of many infectious diseases.

The significance of zoonoses in the emergence of human infections cannot be overstated. The introduction of viruses into human populations, for example, is often the result of human activities, such as agriculture, that cause changes in natural environments. These changes may place humans in contact with infected animals or with arthropod vectors of animal diseases, thereby increasing the chances of human infection. Argentine hemorrhagic fever, a natural infection of rodents, emerged as a result of an agricultural practice placing humans in close proximity to the rodents. Marburg, Machupo, Hantaan, and Rift Valley fever viruses are also of zoonotic origin, as, arguably, is human immunodeficiency virus (HIV). Yellow fever, whose natural cycle of infection takes place in a jungle habitat and involves monkeys and mosquitoes in tropical areas of Africa and South America, is probably an ancient zoonosis. Jungle yellow fever occurs when humans interpose themselves in the natural cycle and are bitten by infected mosquitoes. Yet there is also urban yellow fever, in which the same virus is transmitted among

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