Infectious agent

In biology, a pathogen (Greek: πάθος pathos "suffering, passion" and -γενής -genēs "producer of") in the oldest and broadest sense is anything that can produce disease; the term came into use in the 1880s. Typically the term is used to describe an infectious agent such as a virus, bacterium, prion, a fungus, or even another micro-organism.

There are several substrates including pathways where the pathogens can invade a host. The principal pathways have different episodic time frames, but soil contamination has the longest or most persistent potential for harboring a pathogen. Diseases caused by organisms in humans are known as pathogenic diseases.

Pathogenicity is the potential disease-causing capacity of pathogens. Pathogenicity is related to virulence in meaning, but some authorities have come to distinguish it as a qualitative term, whereas the latter is quantitative. By this standard, an organism may be said to be pathogenic or non-pathogenic in a particular context, but not "more pathogenic" than another. Such comparisons are described instead in terms of relative virulence. Pathogenicity is also distinct from the transmissibility of the virus, which quantifies the risk of infection.

A pathogen may be described in terms of its ability to produce toxins, enter tissue, colonize, hijack nutrients, and its ability to immunosuppress the host.

It is common to speak of an entire species of bacteria as pathogenic when it is identified as the cause of a disease (cf. Koch's postulates). However, the modern view is that pathogenicity depends on the microbial ecosystem as a whole. 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. 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".

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