A 17th century Dutch merchant named Antoni van Leeuwenhoek was the first person to see a living microbe. To judge the quality of some cloth, he made a powerful magnifying lens. Leeuwenhoek looked at other things too-a hair from his beard, a flea, and a drop of rain. The water, he discovered, teemed with "little animals" —what we now refer to as microbes.

It was 200 years before researchers realized that some of these microbes could cause disease. Louis Pasteur, who was studying the microbes that cause wine to ferment, reasoned that microbes might also be responsible for illnesses.

By the 1880s, the German scientist Robert Koch identified the bacteria that cause anthrax, cholera, and tuberculosis. In his work on anthrax, Koch developed a method for linking particular microbes with the diseases they cause. This included finding the microbe in every case of the disease, growing the microbe in a lab dish, and testing the effect of the lab-grown microbe in animals.