Pandemics have a way of changing the world.
The Plague of Justinian hit the Mediterranean area in the 500s, not only killing millions but crucially weakening the Byzantine Empire and helping ruin its plans to reconquer western Europe from the barbarians.
The Black Death of the 1300s wiped out over 100 million people in Europe and Asia. It sped the decline of the feudal system, engendered persecution of Jews, lepers and gypsies, and created a better economic life for the peasantry whose labour was now much more valuable.
The Spanish flu of 1918-20 and its 50 million deaths helped the Allies defeat Germany in the First World War, increased government intervention in public health, and spurred the eugenics movement and the drive to rid society of its weakest members.