Since I am basically out of ideas, I have decided to steal topics from Waldemar Heckel’s midterm essay topics.
The ancient sources suggest two possibly causes of death, both of which are likely wrong.
1) He drank himself to death. While drink may have impacted his later life, immediate cause of death was probably pneumonia and therefore not alcohol caused.
2) Someone had him poisoned. Like I said, pneumonia.
3) — this is what I think– Alexander campaigned continuously for fifteen years, marching with his men and receiving numerous wounds. The would that proved to be fatal was an arrow received jumping into an enemy fortress all alone. This arrow pierced his lung and took quite some time to heal. When pneumonia struck about a year and a half later his body simply could not cope.