The main cause of unexplained fever in children is infection, which causes around 50 percent of all fevers of unknown origin, according to the University of Maryland Medical Center. Malignant or inflammatory diseases account for roughly 5 to 10 percent of unexplained fever in children.
Doctors classify a fever as having unknown origin if it is over 100 degrees Fahrenheit and extensive diagnostic testing does not reveal its cause, notes the University of Maryland Medical Center. Treatment options for unexplained fever vary and may include the administration of fever-reducing medications and antibiotics. Drinking sufficient fluids and resting can also be helpful.