||This article needs additional citations for verification. (September 2008)|
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its virulence factors. The noun virulence derives from the adjective virulent. Virulent can describe either disease severity or a pathogen's infectivity. The word virulent derives from the Latin word virulentus, meaning "a poisoned wound" or "full of poison."
In an ecological context, virulence can be defined as the host's parasite-induced loss of fitness. Virulence can be understood in terms of proximate causes—those specific traits of the pathogen that help make the host ill—and ultimate causes—the evolutionary pressures that lead to virulent traits occurring in a pathogen strain.
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 enzymes. These proteins are coded for by genes in chromosomal DNA, bacteriophage DNA or plasmids. In certain bacteria, mobile genetic elements and horizontal gene transfer. Therefore, strategies to combat certain bacterial infections by targeting these specific virulence factors and mobile genetic elements have been proposed. Bacteria use quorum sensing to synchronise release of the molecules. These are all proximate causes of morbidity in the host.
Methods by which bacteria 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 correlate 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 viruses 
Virus 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 HIV, which causes AIDS, have mechanisms for evading host defenses. HIV infects T-Helper Cells, which leads to a reduction of the adaptive immune response of the host and eventually leads to an immunocompromised state. 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. The term "neurovirulent" is used for viruses such as rabies and herpes simplex which can invade the nervous system and cause disease there.
According to evolutionary medicine, optimal virulence increases with horizontal transmission (between non-relatives) and decreases with vertical transmission (from parent to child). This is because the fitness of the host is bound to the fitness in vertical transmission but is not so bound in horizontal transmission.
The pathogen population can evolve once it is in the host. There are three main hypotheses about why a pathogen evolves as it does. These three models help to explain the life history strategies of parasites, including reproduction, migration within the host, virulence, etc. The three hypotheses are the Trade-Off Hypothesis, the Short-Sighted Evolution Hypothesis, and the Coincidental Evolution Hypothesis. All of these offer ultimate explanations for virulence in pathogens.
Trade-off hypothesis 
At one time, some biologists argued that pathogens would tend to evolve toward ever decreasing virulence because the death of the host (or even serious disability) is ultimately harmful to the pathogen living inside. For example, if the host dies, the pathogen population inside may die out entirely. Therefore, it was believed that less virulent pathogens that allowed the host to move around and interact with other hosts should have greater success reproducing and dispersing.
But this is not necessarily the case. Pathogen strains that kill the host can increase in frequency as long as the pathogen can transmit itself to a new host, whether before or after the host dies. The evolution of virulence in pathogens is a balance between the costs and benefits of virulence to the pathogen. For example, Mackinnon and Read (2004) and Paul et al. (2004)  studied the malaria parasite using a rodent and chicken model respectively and found that there was trade-off between transmission success and virulence as defined by host mortality.
Short-sighted evolution hypothesis 
Short-sighted evolution suggests that the traits that increase reproduction rate and transmission to a new host will rise to high frequency within the pathogen population. These traits include the ability to reproduce sooner, reproduce faster, reproduce in higher numbers, live longer, survive against antibodies, or survive in parts of the body the pathogen does not normally infiltrate. These traits typically arise due to mutations, which occur more frequently in pathogen populations than in host populations, due to the pathogens' rapid generation time and immense numbers. After only a few generations, the mutations that enhance rapid reproduction or dispersal will increase in frequency. The same mutations that enhance the reproduction and dispersal of the pathogen also enhance its virulence in the host, causing much harm (disease and death). If the pathogen's virulence kills the host and interferes with its own transmission to a new host, virulence will be selected against. But as long as transmission continues despite the virulence, virulent pathogens will have the advantage. So, for example, virulence often increases within families, where transmission from one host to the next is likely, no matter how sick the host. Similarly, in crowded conditions such as refugee camps, virulence tends to increase over time since new hosts cannot escape the likelihood of infection.
Coincidental evolution hypothesis 
Some forms of pathogenic virulence did not co-evolve with the host. For example, tetanus is caused by the soil bacterium Clostridium tetani. After C. tetani bacteria enter a human wound, the bacteria may grow and divide rapidly, even though the human body is not their normal habitat. While dividing, C. tetani produce a neurotoxin that is lethal to humans. But it is selection in the bacterium's normal life cycle in the soil that leads it to produce this toxin, not any evolution with a human host. The bacterium finds itself inside a human instead of in the soil by mere happenstance. We can say that the neurotoxin is not directed at the human host.
More generally, the virulence of many pathogens in humans may not be a target of selection itself, but rather an accidental by-product of selection that operates on other traits, as is the case with antagonistic pleiotropy
See also 
- MeSH - Medical Subject Headings, Karolinska Institute, 13 April 2010
- Compact Oxford English Dictionary virulent
- A Latin Dictionary virulentus
- Encyclopædia Britannica Online, 25 May 2009. "plant disease development"
- Keen, E. C. (2012). "Paradigms of pathogenesis: Targeting the mobile genetic elements of disease". Frontiers in Cellular and Infection Microbiology 2. doi:10.3389/fcimb.2012.00161.
- Madigan M, Martinko J (editors) (2006). Brock Biology of Microorganisms (11th ed.). Prentice Hall. ISBN 0-13-144329-1.
- Encyclopædia Britannica Online, 2009. "lytic phage"
- Mackinnon, M; A Read (2004). "Virulence in malaria: an evolutionary viewpoint". Philosophical Transactions of the Royal Society of London Series B-Biological Sciences 359 (1446). doi:10.1098/rstb.2003.1414.
- Paul, R; T Lafond, CDM Muller-Graf, S Nithiuthai, PT Brey, JC Koella (2004). "Experimental evaluation of the relationship between lethal or non-lethal virulence and transmission success in malaria parasite infections.". BMC Evolutionary Biology 4. doi:10.1186/1471-2148-4-30.