The Plague.

The plague has probably caused more fear and terror than any other disease in history. It has killed more than two hundred million people. But it also brought about some large changes, such as the end of the Dark Ages and the advancement of clinical research. The plague is caused by a bacteria, Pasteurella pestis. The bacteria is originally carried by infected rats, which are bitten by fleas. The bacteria completely fills the flea's stomach …

…though because full vaccination requires five to seven months to complete. Therefore, it doesn't really help during outbreaks. The plague has touched every continent except Australia and Antarctica. Unlike other infectious diseases, the plague will never be eradicated. It lives in millions of animals and billions of fleas. It can be anywhere; on snowy mountains, hot deserts, dry grasslands, or evergreen forests. It is a disease that can kill a person and destroy a continent.