The germ theory of disease was pioneered by several doctors and scientists, but much of the credit for germ theory is given to Louis Pasteur. His careful, documented experiments helped to establish microbes as life forms.
Pasteur believed that living organisms could only come from other living organisms. This theory is called biogenesis. During this time period, most people believed in spontaneous generation, that living things could be spawned by non-living things. Pasteur believed that tiny organisms existed in the air and affected the things around them. He proved this theory by scientifically observing meat broth for spoilage. His experiments changed the way people thought about germs and disease.
Robert Koch's development of microscope techniques helped promote belief in germ theory because microorganisms could be observed and identified. Koch developed four criteria for demonstrating that a disease is caused by a particular organism; these criteria are known as Koch's postulates.