Koch’s original postulates or a modified version continue to play an essential role in modern epidemiology. Nearly every year, new diseases arise or old ones spread out of their usual range. Notable examples include Lyme disease, AIDS, SARS, West Nile fever, and Whipple disease. Whipple disease is a chronic intestinal infection, which for many years lacked a known causative agent. Recently, investigators felt sure enough of their identification of a bacterium in infected tissues to announce they had finally identified the mystery pathogen, calling it Tropheryma whipplei.