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Quintus Curtius Rufus III 5 iii GÇÿScarcely had he entered the water when suddenly he felt his limbs shiver and stiffen. He went pale, and the vital heat all but left his body.GÇÖ
There seem to be two trains of thought as to the nature of AlexanderGÇÖs malady; Engels suggests a malarial attack on the central nervous system (Classical Philology 73 GÇÿA Note on AlexanderGÇÖs DeathGÇÖ 1978 pp224-8) whereas Schachermayer and Green plump for pneumonia.
The symptoms of malaria caused by the plasmodium falciparum include cramping, fever and insomnia it seems but CurtiusGÇÖ makes great play of the suddenness of the attack, and an equally speedy recovery, probably five days this seems far too quick for either malaria or pneumonia.
It is the shock of the cold waters of the Cydnos on AlexanderGÇÖs overheated body that precipitates the attack. The symptoms of sudden and intense muscle pain, pallor, shortness of breath and clammy cold flesh recall nothing so much as a heart attack or most probably angina.
It seems odd that no one should have suggested this before; perhaps the idea of the super fit hero will not allow the possibility of this condition. Later at 5 ix-x Curtius says GÇÿIndeed, the violence of his illness appeared to have abated simply because Alexander realised how serious the situation was. [10] However his anxiety adversely affected his conditionGǪGÇÖ This too seems to fit a cardiac problem for there the best thing to do is relax and not panic; Alexander comes round among his friends gets a grip but thinking on how helpless he is relapses.
Philip the AcarnanianGÇÖs purge will have been effective for psychological reasons, Alexander was a fit young man and his body would mend itself if allowed to. The initial shock of the drug may well have proved fatal as it seems to have precipitated another seizure but either through narcotics contained therein or AlexanderGÇÖs faith he relaxed and his own body went to work. Five days and perhaps a months convalescent killing and fining would be reasonable for recovery from an angiotic attack brought on by the sudden constriction of a narrow blood vessel in the heart due to immersion in freezing water.

Karl,I never really gave this too much thought. Interesting though isn't it? You've layed out some rather sophisticated medical terms, are you trained in the medical profession? It seems to me, that if Alexander was unknowingly, already on the verge of becoming ill, that this sudden shock might cause him to convulse and fall deathly ill in an acute manner. A simple common cold could be made much worse by ice water immersion, especially after his pores had become dilated for maximum heat release induced by a strenuous workout. later Nicator

I am not medically trained but I have alot of sick friends boom boom; actually quite a few of my associates work in that sphere so the jargon has just rubbed off.

Last week I was watching discovery channel featuring a
documentary about the mystery behind Alexander's death. Various
doctors, physicians and others gave in their opinion and I wanted
to share with you the outcome of this documentary.First they contrasted Alexander to a modern work aholic business
man who is not in good health due to his work obessions, but is
not patient enough to take a couple of days to rest. By the way, I
strongly disagree with this contrast; I truly believe that if Alexander
was living these years, he would be a high official in some
nationwealth organization, as in my opinion, his overall scope was
to have a better world. Returning to the documentary, after going through many possible
murder conspiracies, which were all discarted, all specialists agreed
that Alexander was poisoned. But not intentionally. The end result
was that Alexander was taking medicine to recover from his many
physical and spiritual wounds. Though this medicine, since they
always encorporate a cocktail of drugs, included also a herb that at
that time it was unknown to be poisonous. Alexander kept on
taking the medicine convinced of being healed while in fact it was
slowly and invisibly killing him.On a personal note, if I may, I must say that I always get sad when
reading or writing about Alexander's death. He's my hero! But
thinking about it, isn't a mysterious and an early death the final
fate of any hero?Zen

That sounds alot like the theory promoted by a British policeman here that he poisoned himself accidentaly with hellebore,sadly the ancients were aware of the danger in its use and Plutarch 41 vii even reports that Alexander warned Krateros' doctor about it, even though the letter is probably bogus it still shows that the dangers of hellbore were known.