Given the rapid increase in the number of laboratories around the world that do research on diseases of bioterrorism concern, such as anthrax and plague, and emerging infections, such as SARS and avian influenza, there is an urgent need to prevent pathogenic microbes from escaping accidentally from a lab or being stolen by terrorists and criminals. With a few exceptions, dangerous bacteria and viruses can be isolated from nature, but different strains tend to vary widely in virulence, or the ability to cause illness and death.