The bubonic plague killed an estimated 50 million people in the 14th century. Pandemics aren't a thing of ancient times though; last century saw up to 50 million die of Spanish flu. Even in age of advanced medical care, the body can still be completely at the mercy of microscopic bacteria, viruses, and parasites. Indeed, some of these terrifying bugs, such as MRSA, are in fact the product of our scientifically advanced societies.
This video from Alltime10s shows ten of the most deadly diseases that have the ability to kill you in under 24 hours. Be prepared for hemorrhaging orifices, flesh-eating bacteria and all manner of horrifying symptoms.