Before Robert Koch's discovery, many doctors believed that diseases just happened. They thought bad air or bad luck made people sick. Koch had a different idea. He believed tiny living things called bacteria caused specific diseases. On March 24, 1882, Koch announced that he had found the bacterium that causes tuberculosis, a deadly lung disease. His discovery changed medicine forever. Koch used a microscope to study tissue from sick patients. He developed a method of staining the bacteria with special dyes so they could be seen more clearly. The bacteria were so small that millions could fit on the head of a pin. Koch did not just find the bacteria. He proved they caused the disease by following a set of steps now called Koch's postulates. First, he found the bacteria in every sick patient. Second, he grew the bacteria in a laboratory dish. Third, he injected the lab-grown bacteria into healthy animals, which then became sick. Fourth, he found the same bacteria in the newly sick animals. This logical method became the standard for proving that a specific germ causes a specific disease. Koch won the Nobel Prize in 1905 for his work on tuberculosis.