Sure.
Introduction of a foreign virus or disease, or a mutation in an existing one. Failure of sanitation due to overcrowding. They say that the one single technology that has saved more lives than any other has been the introduction of sanitary sewage disposal--sewers!
Just think, antibiotics, modern surgery, hospitals, Xrays, all take a second place to something the Romans figured out thousands of years ago: don't shit where you eat.
Of course, back in those days, most people believed disease was caused by the gods, as punishment for our sins.