The causes of heatstroke are a handful. The foremost cause is attributed to dehydration. Dehydration simply defined as an absence of water in the body. It is important to note that 75% of the the water in the body is to be found in the cells. The other 25% is to be found in the body's intracellular spaces and all. When there is more water leaving this system than there is water being absorbed, then chances are high that an individual is a likely candidate of dehydration. This is of course followed by a bout of heatstroke on account of the fact that the body lacks the necessary fluid to aide in cooling it down. This is the fast cause of heat stroke.
The second cause of heat stroke is the body's inability to disperse heat for one reason or another. This failure might be attributed to many reasons. Foremost it is important to understand that one way the body deals with matters heat dispersion is via sweating. In an environment where the humidity levels are high, this function might be obstructed. This failure to sweat has the effect of causing an internal heat swell that results in the body's temperature drastically increasing and this could lead to a bout of heatstroke. The second possible scenario where the body is unable to disperse heat on account of an individual being thoroughly robed. The effect of this is that despite the body's attempt to disperse heat it ( heat ) is still retained in the system to the detriment of the individual. These are the two possible scenarios for the body failing to disperse heat - humid conditions, and extreme heat.