Mass killers

Tsetse fly

Mosquitoes

Female mosquitoes feed on human blood to provide protein for their eggs. Some species infect humans with the disease malaria. This kills up to 1.5 million per year and affects another 300 million!

When an infected tsetse fly bites its victim, it injects  a tiny parasite into the bloodstream. This eventually invades the brain, causing drowsiness and fever known as "sleeping sickness", which kills 40,000 people per year. 

Fleas

Fleas on rats carried the potentially fatal disease known as the bubonic plague. The black death was an epidemic of this plague that wiped out some 75 million people worldwide between 1347 and 1351!