The repeated visitations of the Black Death to Europe during the late Middle Ages were certainly traumatic. As many as one-third of the population died in a single visitation, agonizingly. The disease seemed to come from nowhere and depart as mysteriously. No wonder people looked for a transcendental cause.
But, of course, God or the Devil had nothing to do with it. The culprit was a bacterium called Yersinia pestis. The disease is carried from rats to humans by fleas, that feed first on the infected blood of rats and then bite humans.
When a cell of the human immune system encounters an invading pathogen -- a virus or bacterium -- it sends out a chemical call for help to other immune cells. The summoned cells, together with the summoner, gang up on the pathogen and take it apart.
Yersinia pestis has evolved a brilliant strategy for thwarting this defensive response. The bacteria hovers just outside an immune cell and shoots into the cell a protein called YopJ. This magic bullet instantly dismantles the immune cell's ability to send out the call for help. Effectively silenced and on its own, the immune cell is now easy work for Yersinia pestis's other destructive proteins.
Remember the film Where Eagles Dare? Richard Burton and a small band of Allied agents infiltrate a Nazi castle stronghold in Bavaria during World War II. Zap, zap, with a silenced revolver, the invaders kill off Nazi sentries one by one before they can raise the alarm. By the time the Germans finally manage to get their defensive act together, the castle has been pretty much blasted apart and the Allies have accomplished their mission. It is the old Yersinia pestis story all over again.