Everyone knows the prophet died of illness, and many say it was due to poison. Some say it was poison he consumed during te Battle of Khaybar three years earlier, but there are some serious doubts about that theory.
Another theory suggests Aisha may have poisoned him.
This lecture digs into the evidence for and against these theories, focusing primarily on sunni sources.
Podcast version: podcast.lifeofprophetmuhammad.com