Yersinia pestis bacteria. Bubonic Plague ravaged Europe in the 15th century, wiping out one third of the population. Symptoms include a large bluish-black cyst, and a blackening of the skin, which earned it the name the Black Death. Carried by fleas, it became prominent when fear of witches caused the large-scale elimination of cats, allowing rats to run rampant. Variations include an airborne variety that is carried by the mucus in a victim's cough. There are still cases today but modern anti-biotics can effectively treat the disease. Resistent strains of this bacteria are an extremely large threat. Scientists are investigating the possibility that decendents of plague survivors are resistent to AIDS because the Yersinia bacteria attacks the same macrophages that the HIV does.