BIOLOGICAL AGENTS AND DISEASE TRANSMISSION
For most of history, people have believed that the transmission of disease is a mysterious phenomenon controlled not by humans but by the gods, witchcraft, or fate. Collective disease, including the sudden epidemics that decimated cities and armies, was a frequent but misunderstood occurrence. Often relying on the ancient Greeks, Western scholars and physicians from the Middle Ages well into the nineteenth century held firm to the notion that “miasma,” the stench of putrefaction, was the source of many epidemics and that changes in the weather or the planets increased the chances of outbreaks.1 One of the positive results of the belief in miasma was the instigation of campaigns to clear cities of garbage, open sewage, standing water, slums, and unhygienic slaughterhouses, measures that significantly reduced the risks of infectious diseases. In both Europe and Asia, public health measures allowed urban economic growth. But whole societies remained vulnerable to devastating epidemics that were explained either fatalistically or by a science still struggling for proof.
Toward the end of the nineteenth century, before any state biological weapons program, medical scientists discovered microorganisms and made great strides toward understanding that a specific germ can cause a specific disease; that food, water, and personal contact communicate illness; that a pathogen can cycle through different species; and that insects and protozoans play a role in creating epidemics. Once these causal links were understood, humans could methodically control disease outbreaks. It became more possible to protect populations from the great assaults of plague, cholera, diphtheria, smallpox, influenza, and malaria that had swept across nations in previous centuries, hit