The rat flea is the culprit for transmitting the Bubonic plague or black death. The rat flea is a parasite of house and field rats but will seek out other host animals when their current host dies from infection. Bacteria spread through the flea’s bites on the host animal (usually rats), and the infected animal sickens and dies. Rats have long been viewed as the bearer of the plague, but it is actually the fleas they carry that cause the illness. Dead infected animals can also transfer the disease through contact with their bodily fluids. The Black Death, which spread throughout Europe, Asia, and Africa in the 1300s, killed around 50 million people. The Bubonic plague still exists today, but antibiotics are effective in treating the infection if caught early.