<P> The germ theory of disease states that many diseases are caused by microorganisms . These small organisms, too small to see without magnification, invade humans, animals, and other living hosts . Their growth and reproduction within their hosts can cause a disease . "Germ" may refer to not just a bacterium but to any type of microorganisms, especially one which causes disease, such as protista, fungi, virus, prion, or viroid . Microorganisms that cause disease are called pathogens, and the diseases they cause are called infectious diseases . Even when a pathogen is the principal cause of a disease, environmental and hereditary factors often influence the severity of the disease, and whether a potential host individual becomes infected when exposed to the pathogen . </P> <P> The germ theory was proposed by Girolamo Fracastoro in 1546, and expanded upon by Marcus von Plenciz in 1762 . Such views were held in disdain, however, and Galen's miasma theory remained dominant among scientists and doctors . The nature of this doctrine prevented them from understanding how diseases actually progressed, with predictable consequences . By the early nineteenth century, smallpox vaccination was commonplace in Europe, though doctors were unaware of how it worked or how to extend the principle to other diseases . Similar treatments had been prevalent in India from just before 1000 A.D. A transitional period began in the late 1850s as the work of Louis Pasteur and Robert Koch provided convincing evidence; by 1880, the miasma theory was struggling to compete with the germ theory of disease . Eventually, a "golden era" of bacteriology ensued, during which the theory quickly led to the identification of the actual organisms that cause many diseases . Viruses were discovered in the 1890s . </P> <P> The miasma theory was the predominant theory of disease transmission before the germ theory took hold towards the end of the 19th century . It held that diseases such as cholera, chlamydia infection, or the Black Death were caused by a miasma (μίασμα, Ancient Greek: "pollution"), a noxious form of "bad air" emanating from rotting organic matter . Miasma was considered to be a poisonous vapor or mist filled with particles from decomposed matter (miasmata) that was identifiable by its foul smell . The theory posited that diseases were the product of environmental factors such as contaminated water, foul air, and poor hygienic conditions . Such infections, according to the theory, were not passed between individuals but would affect those within a locale that gave rise to such vapors . </P>

Who came up with the germ theory of disease