The microorganisms which cause diseases are called pathogens.
In biology, a pathogen (Greek: πάθος pathos "suffering, passion" and -γενής -genēs "producer of"), infectious agent, or a germ in the oldest and broadest sense is anything that can produce disease; the term came into use in the 1880s.[1][2] Typically the term is used to describe an infectious microorganism or agent, such as a virus, bacterium, protozoa, prion, or fungus.[3][4] The scientific study of these organisms is called microbiology, while the study of disease that may include these pathogens is called pathology.
There are several substrates including pathways where the pathogens can invade a host. The principal pathways have different episodic time frames, but soil has the longest or most persistent potential for harboring a pathogen. Diseases caused by organisms in humans are known as pathogenic diseases.
Pathogenicity
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.
Context-dependent pathogenicity
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".[6]
Related concepts
Virulence
Virulence (the tendency of a pathogen to cause damage to a host's fitness) evolves when that pathogen can spread from a diseased host, despite that host being very debilitated. Horizontal transmission occurs between hosts of the same species, in contrast to vertical transmission, which tends to evolve 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.
Evolutionary medicine has found that under horizontal transmission, the host population might never develop tolerance to the pathogen.[citation needed]
Transmission
Main article: Transmission (medicine)
Transmission of pathogens occurs through many different routes, including airborne, direct or indirect contact, sexual contact, through blood, breast milk, or other body fluids, and through the fecal-oral route.