Scanning electron microscope image of Vibrio cholerae. This is the bacterium that causes cholera. The<unk>  is the currently accepted scientific theory for many<unk> s. It states that microorganisms known as pathogens or "germs" can lead to<unk> . These small organisms, too small to see without magnification, invade humans, other animals, and other living hosts. Their growth and reproduction within their hosts can cause<unk> . "Germ" may refer to not just a bacterium but to any type of microorganism, such as protists or fungi, or even non-living pathogens that can cause<unk> , such as viruses, prions, or viroids.<unk> s caused by pathogens are called infectious<unk> s. Even when a pathogen is the principal cause of a<unk> , environmental and hereditary factors often influence the severity of the<unk> , and whether a potential host individual becomes infected when exposed to the pathogen. Pathogens are<unk> s that can pass from one individual to another, both in humans and animals. Infectious<unk> s are caused by biological agents such as pathogenic microorganisms (viruses, bacteria, and fungi) as well as parasites. Basic forms of<unk>  theory were proposed by Girolamo Fracastoro in 1546, and expanded upon by Marcus von Plenciz in 1762. However, such views were held in disdain in Europe, where Galen's miasma theory remained dominant among scientists and doctors. By the early 19th century, smallpox vaccination was commonplace in Europe, though doctors were unaware of how it worked or how to extend the principle to other<unk> s. A transitional period began in the late 1850s with the work of Louis Pasteur. This work was later extended by Robert Koch in the 1880s. By the end of that decade, the miasma theory was struggling to compete with the<unk> . Viruses were initially discovered in the 1890s. Eventually, a "golden era" of bacteriology ensued, during which the<unk>  theory quickly led to the identification of the actual organisms that cause many<unk> s.
Germ theory of disease
Germ line theory
Genomic imprinting
Imprinted brain hypothesis
Uniparental disomy
Nuclear sexing
Thrifty gene hypothesis
XY sex-determination system