3. Tsetse Fly

Native to tropical Africa, these big, biting flies spread the parasitic infection that causes African sleeping sickness, a disease that is 100 percent fatal without treatment, and the treatment itself is notoriously difficult. There are drugs, but they must be administered with great care, and parasite resistance to them is always a risk. If bitten by an infected tsetse fly, you develop confusion, poor coordination, numbness, and difficulty sleeping. Then you die, most likely. Because the disease is so fatal, recent mitigation efforts have focused on controlling the bugs themselves, which has reduced the number of cases exponentially. In years past, African sleeping sickness was responsible for killing as many as half a million people every year by some estimates. In 2015, fatal cases were estimated at 3,500.