Fever is the temporary increase in the body's temperature, in response to some disease or illness.

A child has a fever when their temperature is at or above one of these levels:

• 100.4 °F (38 °C) measured in the bottom (rectally)
• 99.5 °F(37.5 °C) measured in their mouth (orally)
• 99 °F (37.2 °C) measured under their arm (axillary)

An adult probably has a fever when their temperature is above 99 - 99.5 °F (37.2 - 37.5 °C), depending on what time of the day it is.

See also:

• Temperature measurement
• Heat emergencies

Normal body temperature may change during any given day. It is usually highest in the evening. Other factors that may affect body temperature are.

• In the second part of a woman's menstrual cycle, her temperature may go up by 1 degree or more.
• Physical activity, strong emotion, eating, heavy clothing, medications, high room temperature, and high humidity can all increase your body temperature.

Fever 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. 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.

Brain damage from a fever generally will not occur unless the fever is over 107.6 °F (42 °C). Untreated fevers caused by infection will seldom go over 105 °F unless the child is overdressed or trapped in a hot place.

Febrile seizures do occur in some children. However, most febrile seizures are over quickly, do not mean your child has epilepsy, and do not cause any permanent harm..

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

Almost any infection can cause a fever. Some common infections are

• Respiratory infections such as colds or flu -like illnesses, sore throats, ear infections, sinus infections, infectious mononucleosis , and bronchitis
• Urinary tract infections
• Viral gastroenteritis and bacterial gastroenteritis
• More serious infections such as pneumonia, bone infections (osteomyelitis), appendicitis, tuberculosis, skin infections or cellulitis, and meningitis

Children may have a low-grade fever for one or two days after some immunizations.

Teething may cause a slight increase in a child's temperature, but not higher than 100 °F.

Autoimmune or inflammatory disorders may also cause fevers. Some examples are:

• Arthritis or connective tissue illnesses such as rheumatoid arthritis, systemic lupus erythematosus
• Ulcerative colitis and Crohn's disease
• Vasculitis or periarteritis nodosa

The first symptom of a cancer may be a fever. This is especially true of Hodgkin's disease, non-Hodgkin's lymphoma, and leukemia.

Other possible causes of fever include

• Blood clots or thrombophlebitis
• Medications, such as some antibiotics, antihistamines, some seizure medicines

A simple cold or other viral infection can sometimes cause a high fever (102 - 104 °F, or 38.9 - 40 °C). This does not usually mean you or your child have a serious problem. Some serious infections may cause no fever or even a very low body temperature, especially in infants.

If the fever is mild and have no other problems, you do not need treatment. Drink fluids and rest.

The illness is probably not serious if your child:

• Is still interested in playing
• Is eating and drinking well
• Is alert and smiling at you
• Has a normal skin color
• Looks well when their temperature comes down

Take steps to lower a fever if you or your child are uncomfortable, vomiting, dried out (dehydrated), or not sleeping well. Remember, the goal is to lower, not eliminate, the fever.

When trying to lower a fever:

• Do NOT bundle up someone who has the chills.
• Remove excess clothing or blankets. The room should be comfortable, not too hot or cool. Try one layer of lightweight clothing, and one lightweight blanket for 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, ice, or alcohol rubs. These cool the skin, but often make the situation worse by causing shivering, which raises the core body temperature.

Here are some guidelines for taking medicine to lower a fever:

• Acetaminophen (Tylenol) and ibuprofen (Advil, Motrin) help reduce fever in children and adults. Sometimes doctors advise you to use both types of medicine.
• Take acetaminophen every 4 - 6 hours. It works by turning down the brain's thermostat.
• Take ibuprofen every 6 - 8 hours. DO NOT use ibuprofen in children younger than 6 months old.
• Aspirin is very effective for treating fever in adults. DO NOT give aspirin to children unless your child's doctor so to use it.
• Know how much you or your child weigh, and then always check the instructions on the package.
• In children under 3 months of age, call your doctor first before giving medicines.

Eating and drinking with a fever

• Everyone, especially children, should drink plenty of fluids. Water, popsicles, soup, gelatin are all good choices.
• Do not give too much fruit or apple juice and avoid sports drinks in younger children.
• While eating foods with a fever is fine, foods should not be forced.

Call a doctor right away if your child:

• Is younger than 3 months old and has a rectal temperature of 100.4 °F (38 °C) or higher
• Is 3 -12 months old and has a fever of 102.2 °F (39 °C) or higher
• Is under age 2 years and has a fever that lasts longer than 24 - 48 hours
• Is older and has a fever for longer than 48 - 72 hours
• Has a fever over 105 °F (40.5 °C), unless it comes down readily with treatment and the person is comfortable
• Has other symptoms that suggest an illness may need to be treated, such as a sore throat, earache, or cough
• Has been having fevers come and go for up to a week or more, even if they are not very high
• Has a serious medical illness, such as a heart problem, sickle cell anemia, diabetes, or cystic fibrosis
• Recently had an immunization
• Has a new rash or bruises appear
• Has pain with urination
• Has trouble with their immune system (chronic steroid therapy, after a bone marrow or organ transplant, their spleen previously has been removed, is HIV-positive, or they're being treated for cancer)
• Has recently traveled to a third world country

Call 911 if you or your child has a fever and:

• Is crying and cannot be calmed down (children)
• Cannot be awakened easily or at all
• Seems confused
• Cannot walk
• Has difficulty breathing, even after their nose is cleared
• Has blue lips, tongue, or nails
• Has a very bad headache
• Has a stiff neck
• Refuses to move an arm or leg (children)
• Has a seizure

Call your doctor right away if you are an adult and you:

• Have a fever over 105 °F (40.5 °C), unless it comes down readily with treatment and you are comfortable
• Have a fever that stays at or keeps on going above 103 °F
• Have a fever for longer than 48 - 72 hours
• Have had fevers come and go for up to a week or more, even if they are not very high
• Have a serious medical illness, such as a heart problem, sickle cell anemia, diabetes, cystic fibrosis, COPD, or other chronic lung problems
• Have a new rash or bruises appear
• Have pain with urination
• Have trouble with your immune system (chronic steroid therapy, after a bone marrow or organ transplant, spleen was previously removed, HIV-positive, were being treated for cancer)
• Have recently traveled to a third world country

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.

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.

Legget J. Approach to fever or suspected infection in the normal host. Goldman L, Ausiello D, eds. Cecil Medicine, 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 302.