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When people get sick, they often develop a fever. What is the effect of an increased body temperature on viruses and bacteria in the body? Is it beneficial to the infected body? Importantly, often fever-reducing agents like aspirin are prescribed when people are sick. Doesn't this counteract any benefits of fever?

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Fever is a trait observed in warm and cold-blooded vertebrates that has been conserved for hundreds of millions of years (Evans, 2015).

Elevated body temperature stimulates the body's immune response against infectious viruses and bacteria. It also makes the body less favorable as a host for replicating viruses and bacteria, which are temperature sensitive (Source: Sci Am).

The innate system is stimulated by increasing the recruitment, activation and bacteriolytic activity of neutrophils. Likewise, natural killer cells' cytotoxic activity is enhanced and their recruitment is increased, including that to tumors. Macrophages and dendritic cells increase their activity in clearing up the mess associated with infection.

Also the adaptive immune response is enhanced by elevated temperatures. For example, the circulation of T cells to the lymph nodes is increased and their proliferation is stimulated.

In fact, taking pain killers that reduce fever have been shown to lead to poorer clearance of pathogens from the body (Evans, 2015). In adults, when body temperature reaches 104 oF (40 oC) it can become dangerous and fever reducing agents like aspirin are recommended (source: eMedicine)

Reference
- Evans, Nat Rev Immunol (2015); 15(6): 335–49

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    $\begingroup$ why doe not Ebola gets cured by elevating temp of body of person artificially $\endgroup$ – murmansk Apr 3 '17 at 9:22
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    $\begingroup$ Now, the missing bit is "why are the viruses/bacteria not benefitted by the additional thermal energy"... $\endgroup$ – AnoE Apr 3 '17 at 12:41
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    $\begingroup$ @AnoE There are a lot of examples in nature where larger creatures can ensure more extreme environments than the smaller ones. As an example, many bacteria use simplistic tools to maintain hydrostatic equilibrium with its environment which can fail if the salinity goes up (aka gargling with salt water). Our cells use a more complicated mechanism which can adapt to that salinity. $\endgroup$ – Cort Ammon Apr 3 '17 at 17:11
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    $\begingroup$ That's a good point about fever reducing. In fact, most of the things we think of as "disease symptoms" are actually immune response symptoms, and many methods of treating them end up suppressing the immune response and making the overall experience worse (and longer) rather than better. $\endgroup$ – Mason Wheeler Apr 3 '17 at 17:44
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    $\begingroup$ Most sources attribute elevated temperature as a mechanism to minimize pathogen replication, however, I somewhat am not satisfied with this explanation. Many mesophilic bacteria grow fine even at 45°C. Higher organisms are more susceptible to elevated body temperatures because of their complex organs. I think that elevated temperature is more of a consequence of inflammation than a potentiator. $\endgroup$ – WYSIWYG Apr 4 '17 at 6:33
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Fever normally under hypothalamic heat center's control which stays at limbic system of brain . Hypothalamus sets its own set point 36.4-37.2 in healthy peoples by some molecules named exogenous and endogenous pyrogens, especially PGE2 ,TNF and IL1.

The most important mechanism for fever is directing blood flow from skin to deep vascular pools and preventing heat loss from skin. Thats also cause to cold distal parts of our extremities.

When pathogen microorganisms and their toxins invade our blood, our immune response is being activated. Inflammation process starts which have four components: "Tumor ,Rubor ,Calor ,Dolor". So we experience swelling, redness, fever and pain in other words. Our white blood cells emits prostaglandin and leukotrienes that activating inflammation and those cells to migrate there.

After interact between our defence and microbes, these pyrogens activates hypothalamus heat center. Then temperature set point rises. That activates vasomotor and cortical response. With vasomotor activation, blood flow is directed to deep blood pools and the other side smooth muscles spontaneously contracting generate extra heat. With activation cortical pathways patient feels needing to increase heat or prevent heat loss by e.g wearing more clothes, getting in bed, changing posture to decrease heat losing total area...

So what are all these mechanisms for? Our body resistance increases. Also metabolism speed rises to fight with pathogen. Microorganisms and many toxins have protein components , heat helps denaturation of those proteins and being inactivated. But also our basis contains proteins. So too high temperature is dangerous for our body. When we take analgesics which also decrease fever, thats the purpose to control fever in acceptable intervals. (source:Pubmed) for more read

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  • $\begingroup$ Welcome sinankzlyr to Biology Stack Exchange. Thanks for your contribution to the site. Please can u provide references to support your claims. $\endgroup$ – user237650 Apr 4 '17 at 19:52
  • $\begingroup$ @Mesentery These are all i know, should i still reference for them? Thanks for help. I added one reference. $\endgroup$ – sinankzlyr Apr 4 '17 at 20:08
  • $\begingroup$ Most bacterial proteins are stable even at 50°C; human proteins are more susceptible to heat shock. Whatever may be the beneficial actions of fever but pathogen protein denaturation is surely not one of them. $\endgroup$ – WYSIWYG Apr 5 '17 at 3:44
  • $\begingroup$ @WYSIWYG while proteins may not denature at 40°C, that can be enough to effect protein protein interactions that pathogens rely upon to sustain infections, buying us just enough time for the adaptive immune response to take over. I have to say that I think the first four paragraphs of sinankzlyr's answer do a better job of explaining the how behind temperature elevation, at least in mammals. $\endgroup$ – AMR Apr 6 '17 at 2:47
  • $\begingroup$ @AMR I agree but I don't think there are extensive studies on this. One should compare the HSR of pathogen and host and its effect on the corresponding cells. Whereas I have seen the claim of elevated temperature directly inhibiting pathogen growth so many times without adequate reference. $\endgroup$ – WYSIWYG Apr 6 '17 at 6:06

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