Sharks, vampire bats, crocodiles: the stuff of horror movies. Yes, these animals can kill you, but medical science is turning lemons into lemonade, so to speak. For example, the venom of a cobra:
Well, scientists think that part of what makes venom so nasty is that it suppresses the body's ability to fight back by hampering the immune system's normal healing process. So if you had a disease caused by an overactive immune system, a chemical that slows it down would be the proverbial vigilante crime fighter keeping a corrupt police department in check. This may be why in India cobra venom is considered an arthritis cure -- arthritis is caused by the body's own immune system keeping the sufferer's joints chronically inflamed.
So then you start looking at other even more horrible diseases, like multiple sclerosis and HIV -- both of which involve royal cock-ups in the immune system.