Your body normally cools itself by sweating. During hot weather, especially with high humidity, sweating just isn't enough. Your body temperature can rise to dangerous levels and you can develop a heat illness.
Most heat illnesses occur from staying out in the heat too long. Exercising too much for your age and physical condition are also factors. Older adults, young children and those who are sick or overweight are most at risk.
Drinking fluids, replenishing salt and minerals and limiting time in the heat can help.