This essentially creates 'holes' in the tissue. These abnormally folded proteins are found characteristically in many neurodegenerative diseases as they aggregate the central nervous system and create plaques that damages the tissue structure. They do not contain any DNA or RNA and cannot replicate other than to convert already existing normal proteins to the misfolded state. Prions are misfolded proteins that are transmissible and can influence abnormal folding of normal proteins in the brain. This photomicrograph of the brain tissue shows the presence of the prominent spongiotic changes in the cortex, with the loss of neurons in a case of a variant of Creutzfeldt-Jakob disease (vCJD) Evolutionary biology proposes that many pathogens evolve an optimal virulence at which the fitness gained by increased replication rates is balanced by trade-offs in reduced transmission, but the exact mechanisms underlying these relationships remain controversial. Horizontal transmission occurs between hosts of the same species, in contrast to vertical transmission, which tends to evolve toward symbiosis (after a period of high morbidity and mortality in the population) by linking the pathogen's evolutionary success to the evolutionary success of the host organism. Virulence (the tendency of a pathogen to reduce a host's fitness) evolves when a pathogen can spread from a diseased host, despite the host becoming debilitated. For example, infection of mesenteric lymph glands of mice with Yersinia can clear the way for continuing infection of these sites by Lactobacillus, possibly by a mechanism of 'immunological scarring'. A bacterium may participate in opportunistic infections in immunocompromised hosts, acquire virulence factors by plasmid infection, become transferred to a different site within the host, or respond to changes in the overall numbers of other bacteria present. However, the modern view is that pathogenicity depends on the microbial ecosystem as a whole. It is common to speak of an entire species of bacteria as pathogenic when it is identified as the cause of a disease (cf. ДЄ pathogen may be described in terms of its ability to produce toxins, enter tissue, colonize, hijack nutrients, and its ability to immunosuppress the host. Pathogenicity is also distinct from the transmissibility of a virus, which quantifies the risk of infection. Such comparisons are described instead in terms of relative virulence. By this standard, an organism may be said to be pathogenic or non-pathogenic in a particular context, but not 'more pathogenic' than another. Pathogenicity is related to virulence in meaning, but some authorities have come to distinguish it as a qualitative term, whereas the latter is quantitative. Pathogenicity is the potential disease-causing capacity of pathogens. Not all diseases are caused by pathogens, other causes are, for example, toxins, genetic disorders and the host's own immune system. The principal pathways have different episodic time frames, but soil has the longest or most persistent potential for harboring a pathogen.Дiseases in humans that are caused by infectious agents are known as pathogenic diseases. There are several pathways through which pathogens can invade a host. The scientific study of microscopic organisms, including microscopic pathogenic organisms, is called microbiology, while parasitology refers to the scientific study of parasites and the organisms that host them. However, these animals are usually, in common parlance, referred to as parasites rather than pathogens. Small animals, such as certain worms or insects, can also cause or transmit disease. Typically, the term is used to describe an infectious microorganism or agent, such as a virus, bacterium, protozoan, prion, viroid, or fungus. The term pathogen came into use in the 1880s. A pathogen may also be referred to as an infectious agent, or simply a germ. In biology, a pathogen ( Greek: ПЂО¬ОёОїП‚, pathos 'suffering', 'passion' and -ОіОµОЅО®П‚, -genД“s 'producer of') in the oldest and broadest sense, is any organism that can produce disease. For other uses, see Pathogen (disambiguation).

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