context: A fever in babies can be one of the scariest symptoms for parents, especially when that fever is high or the baby is only a few weeks old. In this article, you'll learn what causes infant fevers and what to do when your baby gets a fever. A fever isn't an illness -- it's considered a symptom of one. A fever usually indicates that the body is fighting an illness and the immune system is functioning. If your baby has a fever, in most instances it means he has probably picked up a cold or other viral infection. Although they are less common in infants, pneumonia, urinary tract infection, ear infection or a more serious infection such as a blood bacterial infection or meningitis, may be causing a fever. Other causes of fever in babies include: Reaction to a vaccination Becoming overheated from being dressed too warmly or spending too much time outside on a hot day One common sign of fever in babies is a warm forehead, although not having a warm forehead doesn't mean that your baby doesn't have a fever. Your baby may also be crankier and fussier than usual. Other symptoms associated with fever in babies include: Poor sleeping Poor eating Lack of interest in play Less active or even lethargic Convulsions or seizures You can take a child's temperature a few different ways, such as via the rectum (rectally), mouth (orally), ear, under the arm (axillary), or at the temples. The American Academy of Pediatrics ( AAP) recommends only using digital thermometers in children. Mercury thermometers should not be used because they pose a risk of mercury exposure and poisoning if they break. Rectal thermometers provide the most accurate temperature readings, and can be easiest to take in an infant. Typically, babies can't hold an oral thermometer in place, and the reading of an ear, temporal, or underarm thermometer are not as accurate. To take a rectal temperature, first make sure the thermometer is clean. Wash it with soap and water or wipe it off with rubbing alcohol. Lay your baby on the belly or on the back with legs bent toward the chest. Apply a little bit of petroleum jelly around the thermometer bulb and gently insert it about 1 inch into the rectal opening. Hold the digital thermometer in place for about two minutes until you hear the "beep." Then gently remove the thermometer and read the temperature. A baby's normal temperature can range from about 97 degrees Fahrenheit up to 100.3 degrees Fahrenheit. Most doctors consider a rectal temperature of 100.4 degrees Fahrenheit or higher as a fever. According to the AAP, call your doctor if your baby: Is under age 3 months and has a fever; if your baby is under 2 months and has a fever, it is considered an emergency. Seek immediate medical care. Is lethargic and not responsive Has problems breathing or eating Is very cranky, fussy or difficult to calm down Has a rash Shows signs of dehydration, such as fewer wet diapers, dry mouth, no tears with crying, or sunken soft spot on the head Has a seizure It can be difficult for doctors to tell whether a newborn has a simple virus (like a cold), or a more serious infection (like UTI, pneumonia or meningitis). That's why doctors will sometimes order special tests (such as blood or urine tests, and/or a chest X-ray and spinal tap) to pinpoint the exact cause of an infant fever, and to look for more serious infections in young babies. If your baby is under age 1 month and has a fever, contact your child's health care provider right away. For older babies, try these tips: Bathe your child with lukewarm water -- always check the temperature of the water on your wrist before washing your baby. Dress your baby in a light layer of clothes. Give your baby enough fluids to avoid dehydration. Those fluids should be breast milk, formula, an electrolyte solution, or water, depending on the baby's age. Contact your baby's health care provider for guidelines. A dehydrated baby may have fewer wet diapers, no tears with crying, or a dry mouth. If your child is older than 6 months and your doctor says it is OK, you can give your baby either children's Tylenol or ibuprofen ( Advil or Motrin). Never give babies aspirin for a fever because of the risk for a rare but potentially dangerous condition called Reye's syndrome. Also, do not give a baby under age 6 months Advil, Motrin, or other medicines that contain ibuprofen. Be sure to ask your doctor about the dosage and read the package instructions before giving your baby a fever-reducing medicine. If you are concerned about your baby's fever, call your health care provider for advice and reassurance.