The most common cause of fever is a viral or bacterial infection, most likely in the respiratory passages (ear, nose, throat, and lungs) or gastro-intestinal (stomach, digestive tract) system. Common childhood illnesses that are likely to cause a fever include croup, bronchitis, strep throat, coxsackie virus, and roseola, as well as garden variety cold and flu viruses.

Children who are attending daycare or are in school are particularly susceptible to infections because they are in contact with other children who may be carrying germs or ill themselves. Children can also be easily infected if someone else in the family is ill. .

An elevated body temperature can also occur for reasons other than illness. Children often run a fever after receiving a vaccination. They can also develop an increased body temperature if they are wearing clothes that are too warm or too tight or if they become overheated while participating in physical activity; this is known as heat-related illness and is not technically a fever. Heatstroke, a more serious form of heat-related illness, is a medical emergency; it can occur when children are trapped or left in very hot places, such as a closed vehicle on a warm day.