Why do epidemics immunodeficiency and autoimmune disease happened?

Because most people who have it don't find out that they do until the later stages of it. While they have it (without them knowing), they probably will still have , which spreads it. And those who DO have it, they might go around getting laid like crazy because they're going to die and they don't give a **** anymore. (Also because they know they already have it so they won't fear contracting a STD -because they already have one!-)