system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Our bodies can adapt to external temperature changes on the fly, but why does an internal temperature change of a few degrees (fever) have such a huge effect on us?
gpt
The hypothalamus controls your core body temperature. The skin controls your external body temperature. Those are related but separate mechanisms. 

Fever is a symptom your body is fighting an infection and are caused by chemicals called pyrogens flowing in the bloodstream. Pyrogens make their way to the hypothalamus in the brain. When pyrogens bind to certain receptors in the hypothalamus, body temperature rises.

Many illness-causing microbes are best adapted to thrive at the body's normal temperature. A fever raises the temperature beyond which certain microbes need to reproduce.

The higher temperature also kicks the immune system into high gear, spurring the rapid production of white blood cells to fight the infection. 

The reason that you feel bad when you have an infection is not directly related to higher body temperature. It's a self preservation mechanism to get you to rest and spend as little energy with other activities. The higher temperature is not the cause, it's a symptom.