And we think we have problems with diseases

According to Wikipedia: In the Late Middle Ages (1340–1400) Europe experienced the most deadly disease outbreak in history when the Black Death, the infamous pandemic of bubonic plague, hit in 1347, killing a third of the human population. Using 7 billion as the current population of the world, that would be a proportionate death toll of around 2.3 billion people within two generations, should a similar situation happen today.