eli5: Why do we “feel” sick?


Whenever i’m sick, i notice that i feel weak, or unmotivated, or similar symptoms.

In: 40

Ditto. I’ve been pondering what a sinking spell is. Or feeling yucky in general. I have post Covid symptoms that exhaust me on certain days and I just feel nauseous.

Your cells in your body are constantly fighting off bad stuff.. if too much bad stuff comes at once your body has trouble fighting it all off at the same time and you start to feel phyaical effects because your body cannot perform optimally and fight the bad stuff off at the same time.. it basically takes energy from you to fight off the bad stuff.. hence you feel bad.

You’re describing [sickness behavior](https://en.wikipedia.org/wiki/Sickness_behavior), which is your brain’s response to inflammation in your body.

When you’re sick, your body has detected something it thinks it a threat. To fight that threat, it is sending out a chemical signal to activate and attract white blood cells. These chemical signals, called *cytokines* (that’s “cell-mover” in Greek – more specifically these are *pro-inflammatory* cytokines), cause all sorts of effects in your body. But as relevant here, they cause effects in your *brain*.

The big cytokine involved here is [IL-6](https://en.wikipedia.org/wiki/Interleukin_6), which stimulates your brain in a few ways. It acts on your hypothalamus to cause your brain to “raise the thermostat” in your body, causing a fever. And it sends signals through the vagus nerve (which connects your brain to your digestive tract, among other things) to trigger sickness behavior.

Sickness behavior, in turn, is a set of adaptations your body uses to get you to:

* be especially sensitive to pain, so you don’t mess with healing processes
* avoid others so you’re not spreading your illness (important in a social animal like humans)
* eat less, which deprives potential invaders of nutrients (iron, in particular, is a big one, and your body actually sucks iron out of your blood when you’re sick)
* move around less, conserving energy for use in fighting the disease

This is why “feeling sick” is a thing, rather than being different for every disease. Feeling “sick” is feeling your body’s systemic inflammatory response, not the direct effects of the disease itself.