Vampire bats sometimes suck blood from humans

While South American vampire bats usually suck blood from livestock, they will occasionally go after humans. To survive, vampire bats must drink a teaspoonful of blood per night. Their saliva contains a powerful blood thinner that is 20 times stronger than any other blood thinner known. This thinner keeps the blood from clotting and makes it easier for the bat to drink it. This substance is used to make a drug for heart attack patients. The medicine is called Draculin.