Eli5 what triggers a fever?


How does our body differentiate between an infection/sickness that causes a fever and what is it in our body that decides whether or not to raise our internal temperature to help fight off something?

In: 1

“What” are called [pyrogens](https://en.wikipedia.org/wiki/Pyrogen_(fever)), “substances that cause a fever”.

Your immune system is composed of cells. Cells can detect, chemically, what’s in the “environment” near them. Inside your body this “environment” is the “just-the-liquid” part of your blood, called plasma.

So your immune cells travel along in your blood and if they detect *any of the substances* that are associated with invasive bacteria, they will go into “attack and defend” mode. Most of the time these substances ARE actually created by bacteria or present on the walls of bacteria, so the immune system cells are correct in attacking and causing a fever.

But sometimes you get exposed NOT to bacteria, but to pieces of (dead) bacteria, pieces of the cellular walls of the bacteria. That’s what pyrogens are, pieces. Your immune system detects them and thinks it “smells” (chemically) the presence of the full live bacterium, so a fever is caused.

Manufacturing anything for medical purposes (medical devices, etc.) we have to be very careful that the devices are sterile (no live bacteria) AND pyrogen-free (no little pieces of destroyed bacteria).