vectors of disease, which can be controlled through the judicious use of pesticides, it is not desirable to kill potential animal reservoirs of disease when the research being pursued requires that the particular animal remain alive. Those who work with imported biological materials need to be aware of the potential for contamination by infectious agents. Although it is generally not done, screening imported laboratory animals, cell lines, and transplantable tumors (including hybridomas used to make monoclonal antibodies) for exotic pathogens probably is the only effective mechanism to reduce the risk of emerging diseases.

Microbes are exceedingly numerous and diverse, but only a small fraction are capable of causing disease in animals or humans. To survive, most microbial species, whether pathogenic or not, must be well adapted to a particular ecological niche and must compete effectively with other micro-organisms. Their small size and high surface-to-volume ratios facilitate rapid growth and extensive impact on their environment. Microbial pathogens can colonize animals, humans, and arthropods because they have acquired (or evolved) a number of genes and gene products that enable them to do so. These gene products are extremely varied, but they include factors involved in transmission from one host to another, in cell-surface attachment and invasiveness, in countering or suppressing specific and nonspecific host immune responses, in persisting or surviving inside and outside a host organism, and in resistance to antimicrobial drugs. Nonpathogens can become pathogens (a rare event), and low-virulence pathogens can become highly virulent through mutation, recombination, and gene transfer.

Because of the relatively small amount of DNA or RNA, or both, that they carry, their rapid growth rate, and large populations, microbial pathogens can evolve and adapt very quickly. These evolutionary mechanisms (Table 2-2) allow them to adapt to new host cells or host species, produce ''new" toxins, bypass or suppress inflammatory and immune responses, and develop resistance to drugs and antibodies. The ability to adapt is required for the successful competition and evolutionary survival of any microbial form, but it is particularly crucial for pathogens, which must cope with host defenses as well as microbial competition. There are, for example, a number of determinants that can exert an influence on viral evolutionary events (Table 2-3). In fact, although hosts can help to drive the evolution of their parasites, the opposite is probably also true (Hamilton et al., 1990). Co-evolution of pathogens and their animal and human hosts will continue to be a challenge to medical science because change, novelty, or "newness" is built into such relationships.