tively, not divine origin, induced disease. In 1546, Fracastorius described the agent of communicable disease as a living contagium vivum that spread by direct contact, by intermediary fomites, or through the air and postulated that these living seeds, passed from one infected animal, produced the same disease in an animal that received them. In 1676, Leeuwenhoek discovered and described bacteria, but bacteriology as a science dates from the middle of the 19th century, a result of the scientific studies of Louis Pasteur. Microbiologists are cognizant of his study disproving the theory of spontaneous generation and his work with cultures and the propagation of bacteria; however, because all of the media were fluid, these methods were not applicable for isolating a single microorganism from mixed cultures. Robert Koch's studies with nutrient gelatin and, later, solid 207