Dehydration
Heat injury.  When it is very hot outside, or when a person is exercising a lot, fluids like water help keep the body cool.  If the person is dehydrated, their body can overheat (it gets too hot and cannot cool itself down).  This can cause heat injury.  The mildest (least serious) form of heat injury is heat cramps (cramping, or pain, in the muscles).  As heat injury gets worse, it becomes heat exhaustion.  If the body keeps overheating and the person does not get medical treatment, heat exhaustion becomes heat stroke.  Heat stroke is a very serious medical emergency.  People with heat stroke can have damage to their organs and even die if they do not get treatment quickly enough.