Abstract
The human race inhabits a world in which it is surrounded by a myriad of different microorganisms—yeasts, bacteria, protozoa, and viruses. Most of these are benign, and some, such as the normal gut flora, play an important part in promoting health via the synthesis of vitamins and stimulation of normal function of gut epithelia. Approximately 0.1% of microbes in our environment have catastrophic effects if they penetrate the epithelial surfaces of the body (Bryson, 2003). History reveals many instances in which armies have been defeated and civilizations have collapsed because of encounters between humans and such microorganisms (Diamond, 1999).