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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.
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
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Virulence". A list of authors is available in Wikipedia.|