The exact cause of Alexander's death is unknown. Historians have debated the issue for centuries, attributing it to poison, malaria, typhoid fever or other maladies. What is agreed upon is that the Macedonian king died in early June 323 BC while suffering a high fever that had lasted ten days.
Why did Alexander get a disease while coming back from India even though he was supposed to be free of all diseases and immortal and son of a Greek god? Since Gods are immortals why demi Gods become mortals? Does the gene to make them immortal are not transferred when they are created? :)