What is Germ Theory?and who proposed?

Germ theory of disease is based on the concept that many diseases are caused by infections with microorganisms, typically only visualized under high magnification. Such microorganisms can consist of bacterial, viral, fungal, or protist species. Although the growth and productive replication of microorganisms are the cause of disease, environmental and genetic factors may predispose a host or influence the severity of the infection. For example, in a host that is immunocompromised (e.g., due to AIDS or old age), an infection may result in more severe outcomes than in individuals who are fully immunocompetent.
The notion that diseases could be spread by “seed-like entities” was first described in the 1500’s by Girolamo Fracastoro and were categorized based on how they could be transmitted. Later, Agostino Bassi in the early 1800’s conducted a series of experiments which demonstrated that a disease afflicting silkworms at the time was caused by a parasite. Bassi theorized that disease in humans and animals was also caused by microorganisms. Bassi’s work served to influence Louis Pasteur, who is accredited with the germ theory of disease following his experiments demonstrating the relationship between microorganisms and disease.