He had just marched to the Indus and back, much of that through desert. He had around five serious wounds in his combats. He was very much in the front fighting. When he was laying up at Babylon there was a fever... Babylon was swampy in those days. Speculation is when he took ill he drank for medicinal reasons and inadvertently drank himself to death.
Well, that all will do it. Still, people manage to drink themselves to death on too much water (yet another case reported just a week or two ago) - so I’ll remain a bit sceptic on the “the alcohol killed him” angle.