Most doctors define a fever as a temperature of 100.4°F (38°C) or higher when taken rectally or 100°F (37.8°C) or higher when taken by mouth. You might get slightly different numbers depending on how you take your child's temperature.

The body has several ways to maintain normal body temperature. The organs that help regulate temperature include the brain, skin, muscle, and blood vessels. The body responds to changes in temperature by:

• Increasing or decreasing sweat production
• Moving blood away from, or closer to, the surface of the skin
• Getting rid of, or holding on to, water in the body
• Seeking a cooler or warmer environment

When your child has a fever, the body works the same way to control the temperature. But it has temporarily reset its thermostat at a higher temperature. The temperature increases for these reasons:

• The body is making certain chemicals called cytokines and mediators. These are made in response to an invasion from a microorganism, malignancy, or other intruder.
• The body is making more macrophages. These are cells that fight when intruders are present in the body. These cells actually "eat up" the invading organism.
• The body is busy trying to make natural antibodies, which fight infection. These antibodies will recognize the infection next time it tries to invade.
• Many bacteria are enclosed in an overcoat-like membrane. When this membrane is disrupted or broken, the contents that come out can be toxic to the body. They cause the brain to raise the temperature.

These conditions can cause a fever:

• Infectious diseases
• Certain medicines or vaccines
• Heat stroke
• Blood transfusion
• Disorders in the brain
• Some kinds of cancer
• Some autoimmune diseases
• Heat stroke

Fever is not an illness. It is a symptom, or sign, that your body is fighting an illness or infection. Fever stimulates the body's defenses. White blood cells and other "fighter" cells are sent out to fight and destroy the cause of the infection.

Children with fevers may become more uncomfortable as the temperature rises. Along with a body temperature greater than 100.4°F (38°C), symptoms may include:

• Your child may not be as active or talkative as usual.
• Your child may seem fussier, less hungry, and thirstier.
• Your child may feel warm or hot. Remember, even if your child feels like they are "burning up," the measured temperature may not be that high.

The symptoms of a fever may look like other health problems. If you are unsure, always check with your child's doctor for a diagnosis.

Use a digital thermometer to check your child's temperature. Don't use a mercury thermometer. There are different kinds and uses of digital thermometers. They include:

• Rectal. For children younger than 3 years, a rectal temperature is the most accurate.
• Forehead (temporal). This works for children age 3 months and older. If a child under 3 months old has signs of illness, this can be used for a first pass. The doctor may want to confirm with a rectal temperature.
• Ear (tympanic). Ear temperatures are accurate after 6 months of age, but not before.
• Armpit (axillary). This is the least reliable but may be used for a first pass to check a child of any age with signs of illness. The doctor may want to confirm with a rectal temperature.
• Mouth (oral). Don't use a thermometer in your child's mouth until they are at least 4 years old.

Use a rectal thermometer with care. Follow the product maker's directions for correct use. Insert it gently. Label it and make sure it's not used in the mouth. It may pass on germs from the stool. If you don't feel okay using a rectal thermometer, ask the doctor what type to use instead. When you talk with any doctor about your child's fever, tell them which type you used.

Below is when to call the doctor if your child has a fever. Your child's doctor may give you different numbers. Follow their instructions.

When to contact a doctor about your child's fever

For a baby under 3 months old:

• First, ask your child's doctor how you should take the temperature.
• Rectal or forehead: 100.4°F (38°C) or higher
• Armpit: 99°F (37.2°C) or higher
• A fever of ___________as advised by the doctor

For a child age 3 months to 36 months (3 years):

• Rectal or forehead: 102°F (38.9°C) or higher
• Ear (only for use over age 6 months): 102°F (38.9°C) or higher
• A fever of ___________ as advised by the doctor

In these cases:

• Armpit temperature of 103°F (39.4°C) or higher in a child of any age
• Temperature of 104°F (40°C) or higher in a child of any age
• A fever of ___________ as advised by the doctor

In children, a fever that is making them uncomfortable should be treated. Treating your child's fever will ease discomfort linked to the fever. It will not help the body get rid of the infection any faster. It will also not reduce the risk of your child having a seizure from the fever (called a febrile seizure).

Children between the ages of 6 months and 5 years can develop febrile seizures. If your child does have a febrile seizure, there is a chance that the seizure may occur again. Febrile seizures rarely cause long-term health problems and are rarely linked to adult epilepsy or seizures. The febrile seizure itself will likely not need to be treated. But the cause of the fever is usually an infection and may need treatment.

Give your child an antifever medicine, such as acetaminophen or ibuprofen. Don't give your child aspirin. It has been linked to a serious, potentially fatal disease, called Reye syndrome.

Other ways to reduce a fever:

• Dress your child lightly. Excess clothing will trap body heat and cause the temperature to rise.
• Encourage your child to drink plenty of fluids, such as water, juices, or popsicles.
• Give your child a lukewarm bath. Do not allow your child to shiver from cold water. It can raise the body temperature. Never leave your child unattended in the bathtub.
• Don't use alcohol baths.