Seems like I read somewhere he died of a lingering infection caused by a severe chest wound.
I think he caught an arrow in the chest while storming a fortified city that was blocking his fleet’s move down the Indus to the ocean. There was also a long overland march through a desert while the fleet moved parallel to the line-of-march. That couldn’t have been good for a sucking chest wound. Plus you can bet everyone had malaria.
He had two or three life-threatening wounds, including the last one during an attack on some city on the Indus River. But it took him and his land forces months and months to get home from there, and they crossed some awful terrain to do it. And he was as strong when they arrived in Mesopotamia as when he’d crossed the Bosphorus. He wasn’t as young though. :’)