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Virulence
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<p>Virulence is a <a href="/facts/Pathogen/axMXtV1M">pathogen</a>'s or <a href="/facts/Microorganism/pysJYcE2">microorganism</a>'s ability to cause damage to a host. </p><p>In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its <a href="/facts/Host_(biology)/L6mop0kh">host</a>. The <a href="/facts/Pathogen/axMXtV1M">pathogenicity</a> of an organism—its ability to cause <a href="/facts/Disease/55eeQlGL">disease</a>—is determined by its <a href="/facts/Virulence_factor/jw099pff">virulence factors</a>. In the specific context of <a href="/facts/Gene-for-gene_relationship/mwjKBXxW">gene for gene</a> systems, often in plants, virulence refers to a pathogen's ability to infect a resistant host. Virulence can also be transferred using a <a href="/facts/Plasmid/yf59JVlA">plasmid</a>. </p><p>The <a href="/facts/Noun/SsnDNfkX">noun</a> <i>virulence</i> (<a href="/facts/Latin/pM3Kge85">Latin</a> noun <i>virulentia</i>) derives from the <a href="/facts/Adjective/0ohBjtmT">adjective</a> <i>virulent</i>, meaning disease severity. The word <i>virulent</i> derives from the Latin word <i>virulentus</i>, meaning "a poisoned wound" or "full of poison". The term <i>virulence</i> does not only apply to viruses. </p><p>From an <a href="/facts/Ecology/YE8upuJU">ecological</a> standpoint, virulence is the loss of <a href="/facts/Fitness_(biology)/MYpAGGey">fitness</a> induced by a parasite upon its host. Virulence can be understood in terms of <a href="/facts/Proximate_and_ultimate_causation/gMJzgPUg">proximate causes</a>—those specific traits of the pathogen that help make the host ill—and <a href="/facts/Proximate_and_ultimate_causation/gMJzgPUg">ultimate causes</a>—the evolutionary pressures that lead to virulent traits occurring in a pathogen strain. </p>