Justinian's Plague, long thought to be the result of infection with Yersinia pestis, was responsible for killing approximately 50 million individuals in 541. Justinian's plague represented the 1st pandemic of plague and was followed about 800 years later by the more famous 2nd plague pandemic, The Black Death.
In 576, about 35 years following this outbreak, the Roman Empire fell and some historians credit the plague's decimation of the population with weakening the already ailing Empire to the point where it was unable to fend off barbarian attacks (see Justinian's Flea).
Using a remnant of a tooth found in a burial pit in Germany that dates from the time of Justinian, a research team was able to extract the DNA of the plague bacillus from dental pulp confirming that Y.pestis indeed was the culprit organism.
It's interesting to imagine what the fate of the world would have been had this plague not occurred. I tend to think that the Roman Empire was already on a death spiral and the plague may only have served to hasten its fall.
Epidemics and their impact on history are a fascinating topic and one of the reasons the subject provides endless enjoyment for me.