Virulence

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Virulence is either the relative pathogenicity or the relative ability to do damage to the host of an infectious agent. The term is used mainly for viruses, but it can be more generally applied to parasites or bacteria. From an ecological point of view, virulence can be defined as the host's parasite induced loss of fitness.

Virulent bacteriophage

Virulent phage infect their bacterial hosts and lyse them after intracellular propagation. Rather than integrating their genome in the host genome a production of new phage progeny is started.

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.

Types of virulence factors

Adhesion. Many bacteria must first stick to host cells at the surface of the body. 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 at the surface of host cells.
Colonization factors'. 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 Factors. 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 blockers. 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.
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 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.