I vote the Bubonic plague, its pretty darn scary, and is very easily spread. I mean25 million people died in just under five years between 1347 and 1352 in Europe alone and it continued to rise every spring after that when the fleas that carried the disease became un-dormant. I would have to say AIDS is up there to, because it also easily spred because so many people are uneducated about the spread and cause of the disease in third world countries.