Rivkin, I don’t know where you read about the diseases, but it is true that it must have been a great problem for them. In ‘The Travels of Ibn Battuta AD 1325-1354’ he writes: “When the Sultan reached the land of Tiling on his way to engage the Sharif in the province of Ma’bar, he halted at the city of Badrakut, capital of the province of Tiling which is at a distance of three months march from the land of Ma’bar. At that moment a pestilence broke out in his army and the greater part of the perished; there died black slaves, the mamluks troopers, and great amirs such as malik Dawlat-Shah, whom the Sultan used to address by the name of uncle, and such as the amir ‘Abdallah al-Harawi, whose story has been related in the first voyage. When the Sultan saw what befallen the army he returned to Dawlat Abad.”
Sultan Muhammad Ibn Tughluq almost lost his Sultanate due to this pestilence, but this time, due to good fortune, he only lost Ma’bar.
More about Ibn Battuta here
http://www.silk-road.com/artl/ibn_battuta.shtml
M Eley, you really sound as if you know the problems people are in when overheated, it must however be stressed, that the trained sportsmen you wrote about have been training for a long time before running a marathon or anything of that kind. Part of the armies in the old days were also trained, they were professional soldiers, but a very big part of the armies were not trained, they were taken from the coolness of the bazaars, where they had been sitting trading for years, dressed for war, and sent to the battle field – most were not trained to use weapons and they were not used to the heat. So the heat, dehydration and illnesses of different kinds might very well have taken a big part of the huge armies they had at the time.