Plague of Amwas

And the conquerors weren't immune.

In 639, the Plague of Amwas swept Syria and killed an estimated 25,000 Muslim soldiers, including the great commander Abu Ubaidah, who Caliph Umar had tried to recall to safety.

Bernstein speculates that Abu Ubaidah's death may have contributed to the Sunni-Shia split because it opened the way for Mu'awiyah's rise.

That might be a stretch, though.