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'Virulence' refers to the degree of pathogenicity of a microbe, or in other words the relative ability of a microbe to cause disease.
The word ''virulent'', which is the adjective for virulence, derives from the Latin word ''virulentus'', which means "full of poison." From an ecological point of view, virulence can be defined as the host's parasite induced loss of fitness.

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Virulent bacteria

Virulent virus

Virulent bacteria

The ability of bacteria to cause disease is described in terms of the number of infecting bacteria, the route of entry into the body, the effects of host defense mechanisms, and intrinsic characteristics of the bacteria called virulence factors. Host-mediated pathogenesis is often important because the host can respond aggressively to infection with the result that host defense mechanisms do damage to host tissues while the infection is being countered.
The virulence factors of bacteria are typically proteins or other molecules that are synthesized by protein enzymes. These proteins are coded for by genes in chromosomal DNA, bacteriophage DNA or plasmids.
'Methods by which pathogens cause disease'

★ 'Adhesion'. Many bacteria must first bind to host cell surfaces. Many bacterial and host molecules that are involved in the adhesion of bacteria to host cells have been identified. Often, the host cell receptors for bacteria are essential proteins for other functions.

★ 'Colonization'. Some virulent bacteria produce special proteins that allow them to colonize parts of the host body. ''Helicobacter pylori'' is able to survive in the acidic environment of the human stomach by producing the enzyme urease. Colonization of the stomach lining by this bacterium can lead to Gastric ulcer and cancer. The virulence of various strains of ''Helicobacter pylori'' tends to corellate with the level of production of urease.

★ 'Invasion'. Some virulent bacteria produce proteins that either disrupt host cell membranes or stimulate endocytosis into host cells. These virulence factors allow the bacteria to enter host cells and facilitate entry into the body across epithelial tissue layers at the body surface.

★ 'Immune response inhibitors'. Many bacteria produce virulence factors that inhibit the host's immune system defenses. For example, a common bacterial strategy is to produce proteins that bind host antibodies. The polysaccharide capsule of ''Streptococcus pneumoniae'' inhibits phagocytosis of the bacterium by host immune cells.

★ 'Toxins'. Many virulence factors are proteins made by bacteria that poison host cells and cause tissue damage. For example, there are many food poisoning toxins produced by bacteria that can contaminate human foods. Some of these can remain in "spoiled" food even after cooking and cause illness when the contaminated food is consumed. Some bacterial toxins are chemically altered and inactivated by the heat of cooking.

Virulent virus

Viral virulence factors determine whether infection occurs and how severe the resulting viral disease symptoms are. Viruses often require receptor proteins on host cells to which they specifically bind. Typically, these host cell proteins are endocytosed and the bound virus then enters the host cell. Virulent viruses such as the AIDS virus (HIV) have mechanisms for evading host defenses. HIV causes a loss of T-cells and immunosuppression. Death results from opportunistic infections secondary to disruption of the immune system caused by the AIDS virus. Some viral virulence factors confer ability to replicate during the defensive inflammation responses of the host such as during virus-induced fever. Many viruses can exist inside a host for long periods during which little damage is done. Extremely virulent strains can eventually evolve by mutation and natural selection within the virus population inside a host.
See also Optimal virulence