Fever is not by itself an illness but a defense mechanism.
However, if it rises above 39.4 °C, some actions have to be taken to reduce it. It meant that the body is no longer able to control its inner temperature.
This can either be done by using antipyretics (medications against fever), or by taking a cooling bath. The way of treatment should be defined by a doctor, based on the underlying cause, the age & condition of the patient. Infectious diseases require anti-pathogenic drugs.
By all means, extra attention should be paid to increased water-intake, to compensate for dehydration.
Young children with even low fever require special attention & immediate treatment aiming to reduce the body temperature (cooling bath or cold pack first, before turning to medications).
If an infant younger than 4 months old has rectal temperature above 38 °C , immediately call your doctor or go to an emergency room, because it could be a sign of a potentially life-threatening infection.
The same action must be taken if any child has a fever above 40 °C.