Why do we get fevers? And if they are defending mechanism of our bodies, how can it be lethal?
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Heat kills the infection, but prolonged heat or too high of a fever can effect the body cells which then effect major organs
Ideally you want the the fever to kill the infection as quickly as possible and then drop right back down to normal. Its why when being monitored by health professionals they sometimes take actions to reduce the fever but not remove it completely. They need the fever to do its job without going too far or for too long that it effects the patient.
So technically a healthy immune system should be able to protect you against every possible virus or bacteria.
The problem is time. It takes time for the immune system to find and manufacture the right weapon for the job. And that is what’s lethal. During that time virus or bacteria keep doing it’s thing only being slowed down by your first line defenses that it might overwhelm. So a virus or bacteria can either kill you or damage your immune system enough so that it can’t help you anymore before it manages to get the right weapon.
When that process works and the infection is defeated your immune system creates memory cells. They make this response time much much shorter and that is why you can be immune to a disease after having it.
This is a mechanizm that vaccines exploit, vaccines fake being (for example) a virus and your body initiates these defense mechanisms, looks for the right weapon and creates memory cells. So now when you get the real thing you have much higher chances of survival.
EDIT: Also, if the question was just about the fever as a symptom, your body increases the temperature to hurt and slow down the invaders not adapted to living in such heat conditions. If the infection is very bad your body might increase the temperature all the way to the point where it becomes lethal to itself. Immune system is very destructive not only to invaders but also to the rest of the body. While fighting a war it doesn’t care about collateral damage.
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