There is a distinction to make between fever, when you get sick, and hyperthermia.

When you get sick and get a fever, that is your brain telling your body to heat up in order to increase the function of your immune system. This process is controlled by the hypothalamus in your brain. In medicine, this fever is defined from a temperature of 38°C. Fever is a reaction of thermoregulation.

On the other hand, hyperthermia occurs when environmental variables are responsible for raising your body's temperature, and the body cannot keep up and cool itself in time, thus overheating.