A natural reaction

A fever is not an illness: it's the body's natural reaction to an attack. 

It could be an internal attack (something as common as teething, or very rare like rhumatism) or external (overheating, infection from germs, bacteria or a virus).

A fever doesn't normally have serious consequences; it's very rare for it to be the sole sign of a serious illness or for it to lead to complications. In fact, a fever is a sign that co-exists with other symptoms. You need to take into account any other signs (loss of appetite, mood, muscular problems) so that that you know how to act and whether or not you need to take your child to the doctor.