In 1546, Girolamo Fracastoro Bassi developed the belief that "bacteridia" or "germs of contagion" may be responsible for illness.

In 1674, Anton van Leewenhoek reported his observations of microscopic bacteria.

Pasteur's work with the spoilage of wine led to his belief by 1857 that microbes could also be the cause of disease.

These ideas led to the adoption of the germ theory of disease.

Robert Koch, a German physician, solidified the germ theory by determining the etiological (causal) agent of the disease anthrax.

 Koch's postulates, a set of logical steps for the identification of pathogenic organisms:

1.  The suspected pathogen must be present in all sick organisms, but absent
     from healthy ones.
2.  The suspected pathogen must be isolated and grown outside the body
     of the host.
3.  When the suspected pathogen is introduced to healthy organisms,
     the hosts must contract the disease.
4.  The suspected pathogen must be re-isolated from the newly infected host
     and shown to be the same as the organism isolated from the original hosts.