<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> <P> In Antiquity, the Greek historian Thucydides (c. 460--c. 400 BC) was the first person to state, in his account of the plague of Athens, that diseases could spread from an infected person to others . One theory of the spread of contagious diseases that were not spread by direct contact was that they were spread by "seeds" (Latin: semina) that were present in the air . In his poem, De rerum natura (On the Nature of Things, ca . 56 BC), the Roman poet Lucretius (ca . 99 BC--ca . 55 BC) stated that the world contained various "seeds", some of which could sicken a person if they were inhaled or if they contaminated his food . The Roman statesman Marcus Terentius Varro (116--27 BC) wrote, in his Rerum rusticarum libri III (Three Books on Agriculture, 36 BC): "Precautions must also be taken in the neighborhood of swamps (...) because there are bred certain minute creatures which cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and there cause serious diseases ." The Greek physician Galen (129 AD--ca . 200 / ca . 216 AD) speculated in his On Initial Causes (ca . 175 AD) that some patients might have "seeds of fever". In his On the Different Types of Fever (ca . 175 AD), Galen speculated that plagues were spread by "certain seeds of plague", which were present in the air . And in his Epidemics (ca . 176--178 AD), Galen explained that patients might relapse during recovery from a fever because some "seed of the disease" lurked in their bodies, which would cause a recurrence of the disease if the patients didn't follow a physician's therapeutic regimen . </P>

Who are the significant scientist who formulated the cell theory