 [ Symptoms ] i have experienced severe pain in both my knees feels like a sledge hammer coming down on my knees [ Disease ] a fever is not an illness    it is just a symptom of one usually if your baby has a fever it means he or she has picked up a cold or other viral infection less commonly in infants a fever is a sign of a bacterial infection such as a urinary tract infection more rarely the infection is something serious like meningitis other causes of fever in babies include reaction to a vaccination becoming overheated from being dressed too warmly or spending time outside on a hot day