system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5... How can a infection like chicken pox trigger alopecia?
gpt
Imagine your body is a city that you have to protect from invaders.
You have the city walls (the innate immune system) and the city guards (the adaptive immune system). Each of these city guards has been trained to look for a particular characteristic that no normal person in your city has, say brown hair with a ponytail (this could be something like the spike protein in COVID-19). Now suppose this guard arrests an intruder with brown hair and a ponytail. They are now on much higher alert than before, so much higher that they start arresting anyone with brown hair, or anyone with a ponytail, even if they are innocent citizens. This is how an infection in your body can cause autoimmune disorders. 

The T-cells in your body are sensitive to particular antigens but are also mildly sensitive to many other antigens, sometimes even self-antigens i.e. your body's own cells. What can happen is that during an infection like chickenpox, the T cell that is specific to a chickenpox antigen will be activated making it much more sensitive overall. It can then react to self-antigens that it was only mildly sensitive to before and begin attacking the body's own cells.