God wanted to punish people.
People and living conditions were dirty.
Fleas living on rats, which came to Europe on trade ships.
Armies attacking cities flung dead bodies infected with the disease over the walls.
Soldiers in the Crimea
A flea carrying the yersinia pestis bacteria.
Mankind's sins had infuriated God.
The alignment of the planets was causing disruption.
Earthquakes had caused bad air to pervade the Earth, making people sick.