Whenever i’m sick, i notice that i feel weak, or unmotivated, or similar symptoms.
In: 40
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.
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