Probably the most difficult potential biological weapon to counter is the genetically engineered threat. Although the bioengineering of microorganisms as weapons has been the subject of fiction in recent years (2), unfortunately it has become a reality (3–5). Advances in biology, genetic engineering, and microbiology coupled with a willingness to exploit these sciences for nefarious purposes have made this possible. The number and type of genetic manipulations that might be undertaken to circumvent standard, as well as advanced, laboratory identification of a pathogen are almost limitless. However, selective pressure and natural genetic exchange have combined to create many serious pathogens in nature. Therefore, much can be learned about the development of a deadly pathogen by the study of the process through natural selection. This knowledge might then be used to estimate what might be done in the laboratory and then develop countermeasures for an engineered pathogen.