Germ theory is the scientific theory that holds that infectious disease is transmitted by microscopic disease carrying particles, rather than the competing theory, miasma theory, which holds that infectious disease comes from a variety of environmental factors. Germ theory was first developed in the 18th century and was largely confirmed by the 1890s, primarily due to the work of John Snow, Louis Pasteur, and Robert Koch. It is now largely agreed that bacteria, viruses and parasites account for all infectious diseases that can be passed either from person to person or from a vector to a human being. Germ theory is the underlying scientific theory behind the modern medical specialties of infectiology and epidemiology.
Although transmission of disease in this matter is largely accepted, it should be noted that outside the medical profession, miasma theory does hold some proponents. For example, there is a group of individuals who believe that AIDS is caused by a combination of sexual behaviours rather than by HIV.