It seems to be a side effect of an excessive immune response to a viral infection, but only in certain people with a hyperactive immune system. It's vaguely lik

It seems to be a side effect of an excessive immune response to a viral infection, but only in certain people with a hyperactive immune system. It's vaguely like the severe reaction some people have after a bee sting. The various viruses that trigger it are mostly harmless to most people.