What exactly is fever?

Question

Pyrexia (or fever) is mostly considered just high temperature, most often caused by infections. Especially in this time of the year, influenza infections that cause fever are quite common.

I would presume that the body heats up in order to kill off bacteria (similar as to why we boil water, and cook food). This doesn't really make much sense when it comes to viruses, as they do not have a similar metabolism. Still, one symptom of influenza is pyrexia, so I conclude that pyrexia is triggered in some part of the immune system which does not differentiate between a viral and bacterial infection. I imagine pyrexia to be the body's standard response to an intruder of any kind. Am I right here?

I also do have 2 follow up questions: How does our body increase the temperature? Is there a fail-safe mechanism (the many deaths caused by high temperature make me think there is none)?

For those who listen to R&B: I'm not looking for the song by Peggy Lee (which is great, by the way)

Answer 1

Fever can be a result of many processes. In context of infectious diseases, the fever is triggered by substances released by the immune cells (substances such as interleukin-1 and -6).
Also, pyrexia can be achieved by getting exogenous pyrogens, substances from the bacterial debris that can create the same response. Those substances then interact with one part of the brain that regulates the temperature.
That part of the brain serves as a thermostat, meaning it sets the "default" body temperature higher in these states. By doing that, the body employs mechanisms to raise the temperature, which result with shaking of the muscles, increase in the metabolic rate, sweating etc. What is interesting is that often people with pyrexia or before the onset of pyrexia feel cold (the default temperature is set to higher point, so the organism feels the current temperature as low).
The point of the fever in this context is not fully discovered, but there is evidence that it provides boost to the immune system production capabilities (increased production of cells to fight the infection etc.) and it might hamper the growth of some infectious organisms that are sensitive to temperature changes.

To be hyperconcise - fever implies immune system activation against antigen OR presence of exogenous pyrogens released from some bacteria (not all bacteria).

About failsafe mechanisms - the thermostat here is enough to coordinate between heat production and heat release. If the temperature goes above the "default", then the organism has many ways to release excess heat (sweating, inactivity, reducing of the metabolism etc.).

Best reading on this topic - Guyton et Hall, Textbook of Medical Physiology 13th ed., ch. 74.