Summer temperatures in Texas can climb above 100 degrees Fahrenheit, making heat stroke a big problem. Heat stroke can be fatal in many cases because it happens so quickly -- there is not much time to react.
Let's say that it really is 100 degrees F outside. The human body wants to stay at 98.6 degrees F. The only way to stay at 98.6 is to sweat . By putting moisture on the skin and letting it evaporate, your body can cool itself very effectively and keep its temperature in the proper range. Sweat works really well as long as there is plenty of water in your body -- it takes water to manufacture sweat. If you run out of water, sweat stops and your body rapidly overheats. It turns out that it is extremely easy to run out of water -- your body can produce 0.5 gallons (2 liters) of sweat every hour in a hot environment. Unless you are drinking water at the same rate, you will dehydrate and then stop sweating. Your internal thirst meter often is not sensitive enough when you need that much water (and it has been said that by the time you feel thirsty, you're already dehydrated), so you have to keep drinking regardless of how thirsty you feel.
The other thing that can lead to heat stroke is very high humidity, which keeps sweat from evaporating.