Normal body temperature varies by person, age, activity, and time of day. The average normal body temperature is 98.6°F (37°C).

Normal body temperature varies.

• In children younger than 6 months of age, the daily variation is small.
• In children 6 months to 2 years old, the daily variation is about 1 degree.
• Daily variations gradually increase to 2 degrees per day by age 6.

Body temperature varies less in adults. However, a woman's menstrual cycle can elevate her temperature by 1 degree or more.

Your body temperature is usually highest in the evening. It can be raised by physical activity, strong emotion, eating, heavy clothing, medications, high room temperature, and high humidity. This is especially true in children.

A rectal temperature up to 100.4°F (38°C) may be entirely normal. A rectal temperature of 100.5°F or above should always be considered a fever. Lower temperatures might also be a fever, depending on the person.

Fever is not an illness. Far from being an enemy, it is an important part of the body's defense against infection. Many infants and children develop high fevers with minor viral illnesses. While a fever signals to us that a battle might be going on in the body, the fever is fighting for the person, not against.

Most bacteria and viruses that cause infections in people thrive best at 98.6°F. Raising the temperature a few degrees can give your body the winning edge. In addition, a fever activates the body's immune system to make more white blood cells, antibodies, and other infection-fighting agents.

Many parents fear that fevers will cause brain damage. Brain damage from a fever generally will not occur unless the fever is over 107.6°F (42°C). Many parents also fear that untreated fevers will keep going higher and higher. Untreated fevers caused by infection will seldom go over 105°F unless the child is overdressed or trapped in a hot place.

Some parents fear that fevers will cause seizures. For the great majority of children, this is not the case. However, febrile seizures do occur in some children. Once a child is already known to have a high fever, a febrile seizure is unlikely with the current illness. In any event, simple febrile seizures are over in moments with no lasting consequences.

Although infections are the most common causes of higher than normal body temperature, fevers have a long list of other causes, including toxins, cancers, and autoimmune diseases.

Heatstroke is a particularly dangerous type of high temperature, because the body is not able to stop the temperature from continuing to rise. It can happen when a child is left in a hot car or when you exercise too strenuously without enough to drink.

Unexplained fevers that continue for days or weeks are called fevers of undetermined origin (FUO).

• Acute bronchitis
• AIDS and HIV infection
• Cancer
• Colds or flu-like illnesses
• Collagen vascular disease, rheumatoid diseases, and autoimmune disorders
• Ear infections
• Fever can occur in infants who are overdressed in hot weather or a hot environment
• Hodgkin's disease
• Infectious mononucleosis
• Inflammatory bowel disease
• Juvenile rheumatoid arthritis
• Leukemia
• Medications (such as antibiotics, antihistamines, barbiturates, and drugs for high blood pressure)
• Neuroblastoma
• Non-Hodgkin's lymphoma
• Occasionally, more serious problems like pneumonia, appendicitis, tuberculosis, and meningitis
• Periarteritis nodosa
• Regional enteritis
• Sore throats and strep throat
• Systemic lupus erythematosus
• Ulcerative colitis
• Upper respiratory infections (such as tonsillitis, pharyngitis or laryngitis)
• Urinary tract infections
• Viral and bacterial infections
• Viral gastroenteritis or bacterial gastroenteritis

If the fever is mild and no other problems are present, no medical treatment is required. Drink fluids and rest. If a child is playful and comfortable, drinking plenty of fluids, and able to sleep, fever treatment is not likely to help.

Take steps to lower a fever if you or your child is uncomfortable, vomiting, dehydrated, or having difficulty sleeping. The goal is to lower, not eliminate, the fever.

When trying to reduce a fever:

• Do NOT bundle up someone who has the chills.
• Remove excess clothing or blankets. The environment should be comfortably cool. For example, one layer of lightweight clothing, and one lightweight blanket to sleep. If the room is hot or stuffy, a fan may help.
• A lukewarm bath or sponge bath may help cool someone with a fever. This is especially effective after medication is given -- otherwise the temperature might bounce right back up.
• Do NOT use cold baths or alcohol rubs. These cool the skin, but often make the situation worse by causing shivering, which raises the core body temperature.
• Drink cool liquids, as tolerated.

Here are some guidelines for taking medicine:

• Acetaminophen and ibuprofen help reduce fever in children and adults.
• Take acetaminophen every 4 - 6 hours. It works by turning down the brain's thermostat. Take ibuprofen every 6 - 8 hours. Like aspirin, it helps fight inflammation at the source of the fever. Sometimes doctors advise you to use both types of medicine. Ibuprofen is not approved for use in children younger than 6 months old.
• Aspirin is very effective for treating fever in adults. Do NOT give aspirin to children unless specifically directed by your doctor.
• Fever medicines come in different concentrations, so always check the instructions on the package.
• Don't use any medicine to reduce fever in children under 3 months of age without first contacting a health care provider.

If someone has heat exhaustion or heat stroke, remove the person from the warm area. Sponge the person with tepid water. Place ice packs in the armpits, behind the neck, and in the groin. Give fluids if the person is alert. Seek medical attention. If heat illness is causing the fever, medicines may not lower the body temperature and may even be harmful.

Call a doctor right away if:

• A baby younger than 90 days old has a rectal temperature of 100.4°F (38°C) or higher.
• A baby 3 to 12 months old has a fever of 102.2°F (39°C) or higher.
• A child under age 2 years has a fever that lasts longer than 24 to 48 hours.
• A fever lasts longer than 48 to 72 hours in older children and adults.
• Anyone has a fever over 105°F (40.5°C), unless it comes down readily with treatment and the person is comfortable.
• There are other worrisome symptoms. For example, irritability, confusion, difficulty breathing, stiff neck, inability to move an arm or leg, or first-time seizure.
• There are other symptoms that suggest an illness may need to be treated, such as a sore throat, earache, or cough.
• You think you may have incorrectly dosed acetaminophen or ibuprofen.

Your doctor will perform a physical examination, which may include a detailed examination of the skin, eyes, ears, nose, throat, neck, chest, and abdomen to look for the cause of the fever. Your doctor may ask questions such as:

• How long has the fever lasted?
• Is it increasing? Is it increasing rapidly?
• Has the fever gone away?
• How many days did it take for the fever to go away?
• Do you have alternating chills and fever?
• How frequently does it alternate (days, hours)?
• Did it occur within four to six hours after exposure to something that you might be allergic to?
• Does the fever follow an up and down pattern (is it high, then lower, then high)?
• Did it develop suddenly?
• Does the temperature go up and down suddenly (spike), or does it change slowly?
• Does it go away and then come back again daily?

Treatment depends on the duration and cause of the fever, and on other accompanying symptoms.

The following diagnostic tests may be performed:

• Blood studies, such as a CBC or blood differential
• Urinalysis
• X-ray of the chest

American College of Emergency Physicians Clinical Policies Subcommittee on Pediatric Fever. Clinical policy for children younger than three years presenting to the emergency department with fever. Ann Emerg Med. 2003;42(4):530-545.

Roth J, Rummel C, Barth SW, et al. Molecular aspects of fever and hyperthermia. Neurol Clin. 2006;24(3):421-439.

Goldman L, Ausiello D, eds. Cecil Medicine, 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007.

Behrman RE, Kliegman RM, Jenson HB, eds. Nelson Textbook of Pediatrics. 17th ed. Philadelphia, Pa: WB Saunders; 2004:839-841.

This article uses information by permission from Alan Greene, M.D., © Greene Ink, Inc.