From Wikipedia

Alexander The Great's Mysterious Death

Final days

On either 10 or 11 June 323 BC, Alexander died in the palace of Nebuchadnezzar II, in Babylon at the age of 32. Plutarch gives a lengthy account of the circumstances of his death, echoed (without firm dates) by Arrian. Roughly 14 days before his death, Alexander entertained his admiral Nearchus, and then, instead of going to bed, spent the night and next day drinking with Medius of Larissa. After this, and by 18 Daesius (a Macedonian month) he had developed a fever, which then grew steadily worse. By 25 Daesius, he was unable to speak. By 26 Daesius, the common soldiers had become anxious about his health, or thought he was already dead. They demanded to see him, and Alexander's generals acquiesced. The soldiers slowly filed past him, whilst Alexander raised his right hand in greeting, still unable to speak. Two days later, on 28 Daesius (although Aristobolus's account says it was 30 Daesius), Alexander was dead. Conversely, Diodorus recounts that Alexander was struck down with pain after downing a large bowl of unmixed wine in honour of Hercules, and (rather mysteriously) died after some agony, which is also mentioned as an alternative by Arrian, but Plutarch specifically refutes this claim.

Possible causes
Poison

Given the propensity of the Macedonian aristocracy to assassination, it is scarcely surprising that allegations of foul play have been made about the death of Alexander. Diodorus, Plutarch, Arrian and Justin all mention the theory that Alexander was poisoned. Plutarch dismisses it as a fabrication, but both Diodorus and Arrian say that they only mention it for the sake of completeness. The accounts are nevertheless fairly consistent in designating Antipater, recently removed from the position of Macedonian viceroy, and at odds with Olympias, as the head of the alleged plot. Perhaps taking his summons to Babylon as a death sentence in waiting, and having seen the fate of Parmenion and Philotas, Antipater arranged for Alexander to be poisoned by his son Iollas, who was Alexander's wine-pourer. There is even a suggestion that Aristotle may have had a hand in the plot. Conversely, the strongest argument against the poison theory is the fact that twelve days had passed between the start of his illness and his death; in the ancient world, such long-acting poisons were probably not available.

Natural causes

Several natural causes (diseases) have been suggested as the cause of Alexander's death; malaria or typhoid fever are obvious candidates. A 1998 article in the New England Journal of Medicine attributed his death to typhoid fever complicated by bowel perforation and ascending paralysis, whereas another recent analysis has suggested pyrogenic spondylitis or meningitis as the cause. Other illnesses could have also been the culprit, including acute pancreatitis or the West Nile virus. Natural-cause theories also tend to emphasise that Alexander's health may have been in general decline after years of heavy drinking and his suffering severe wounds (including one in India that nearly claimed his life). Furthermore, the anguish that Alexander felt after Hephaestion's death may have contributed to his declining health.

The most probable cause of Alexanders death is however, the result of overdosing on medicine made from Hellebore, deadly in large doses. The very few things we do know about his death, can today be explained only with accidental hellebore-poisoning.

An Astronomical diary (c. 323–322 BC) recording the death of Alexander (British Museum, London)