1. The Sun.
Sure, many people will get a sunburn after they spend a summer day at the beach sans sunscreen, but for people with one of the handful of different allergies to the sun, it’s a whole other animal. Itchy red rashes and hives are common, and headaches and blisters aren’t unheard of. One rare, genetic condition is porphyria, known as the “Vampire Disease,” where exposure to the sun can actually cause paralysis and psychosis.