It is called a fever when the core temperature of the body is too high. The best way to measure (eh, estimate) this is to stick a thermometer in Uranus. Because adults are more capable to refuse such a treatment, they are often temp-ed under the tongue or in the armpit. That is even more of an estimate. Today the temperature inside the ear canal is measured with an IR thermometer.
Adults are less capable to shed heat generated inside the body than children because adults have less body surface per kg body mass. Shedding of heat can be supported by a fan, a wet sponge, wet sheets, or even a bathtub filled with ice (Haagen-Dazs?). In adults a fever of 39.4 degrees Celcius becomes dangerous. With children that is over 40 degrees Celcius (104 F). Such temperatures are not uncommon with young children, just stay alert. Though in most cases the temperature drops as fast as it goes up.