Flea
There are many species of flea such as dog fleas, cat fleas, human fleas, northern rat fleas, and the oriental rat flea. During the Middle Ages, the oriental rat flea spread a bacteria that caused the bubonic plague, also known as the Black Death or Black Plague. This was a huge epidemic much more severe than today's bird flu. The oriental rat flea was infected by the bacteria Yersinia pestis.