Editing Virulence (section)

==Virulent viruses==
[[Virus]] virulence factors allow it to replicate, modify host defenses, and spread within the host, and they are toxic to the host.<ref name=Flint2009pp42-47>{{cite book|last1=Flint|first1=S.Jane|last2=Enquist|first2=Lynn W.|last3=Racaniello|first3=Vincent R.|last4=Skalka|first4=Anna Marie|title=Principles of Virology. Vol. II Pathogenesis and Control|year=2009|publisher=ASM|location=Washington, D.C.|isbn=978-1-55581-480-9|pages=42–7|edition=3rd}}</ref>

They 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 [[Endocytosis|endocytosed]] and the bound virus then enters the host cell. Virulent viruses such as [[HIV]], which causes [[HIV/AIDS|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 AIDS. 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 [[Evolution|evolve]] by mutation and [[natural selection]] within the virus population inside a host. The term "[[Neurotropic virus|neurovirulent]]" is used for viruses such as [[rabies]] and [[herpes simplex]] which can invade the [[nervous system]] and cause disease there.{{cn|date=January 2025}}

Extensively studied [[model organism]]s of virulent viruses include [[T4 phage|virus T4]] and other [[T-even bacteriophages]] which infect [[Escherichia coli]] and a number of related [[bacteria]].{{cn|date=January 2025}}

The [[Lytic cycle|lytic life cycle]] of virulent bacteriophages is contrasted by the [[Lysogenic cycle|temperate lifecycle]] of temperate bacteriophages.<ref name=Brock>{{cite book | last1 = Madigan |first1=M. T. |last2=Martinko |first2=J. M. | title = Brock Biology of Microorganisms | edition=11th | publisher = Prentice Hall | year = 2006 | isbn = 978-0-13-144329-7 }}</ref><ref>{{Cite web|title=lytic phage &#124; virus &#124; Britannica|url=https://www.britannica.com/science/lytic-phage|access-date=2023-01-02|website=www.britannica.com|language=en}}</ref><!--Retrieved May 25, 2009, from Encyclopædia Britannica Online-->