What Is It?

A fever is an increase in body temperature above the normal range. However, body temperature varies between people, and also with different levels of activity, and at different times of the day. Modern medical textbooks differ in their definition of the highest normal body temperature, from any temperature above 98.6 degrees Fahrenheit to any temperature above 100.4 degrees Fahrenheit. Fever generally can be defined as an early morning temperature greater than 99 degrees Fahrenheit, or a temperature greater than 100 degrees Fahrenheit at any time of the day.

A part of the brain called the hypothalamus acts as the body’s thermostat. When all is well in the body, the hypothalamus is set at the normal body temperature. Fever develops when the hypothalamus is set to a higher-than-normal temperature. This resetting of the hypothalamus is usually caused by small molecules called pyrogens in the blood. Pyrogens can come from outside the body (external) or can be produced inside the body (internal). External pyrogens include toxins (poisons) produced by infectious viruses or bacteria. Internal pyrogens include abnormal chemicals produced by tumors, and proteins that are released during the normal response of the immune system.

Causes of fever include:

  • Hundreds of types of viruses, bacteria and parasites that cause many illnesses, such as upper respiratory infections, pneumonia, diarrhea, and urinary tract infections
  • Chronic conditions associated with inflammation, such as rheumatoid arthritis
  • Severe trauma, including surgery
  • Reactions to medications or immunizations
  • Certain types of cancers


Common symptoms associated with fevers include sweating, shivering, headaches, muscle aches, poor appetite, rash, restlessness and general body weakness. High fevers can lead to symptoms of mental dysfunction, such as confusion, excessive sleepiness, irritability and convulsions (seizures).

Convulsions triggered by fever (febrile seizures) are common among children younger than 5 years old. These seizures usually occur at the beginning of an illness when the temperature rises rapidly. In infants and toddlers, febrile seizures typically produce muscle rigidity and generalized convulsions that last less than 15 minutes. The febrile seizure usually is followed by a long period of sleep.

Specific symptoms associated with a fever often can provide clues to identify the fever’s cause. For example, a fever accompanied by vomiting and diarrhea may signal gastroenteritis, while a fever associated with coughing, shortness of breath and the presence of grayish-yellow phlegm may indicate pneumonia.


In pinpointing a fever’s cause, your doctor may ask you about:

  • Cold or flu symptoms
  • Abdominal pain, nausea, vomiting, or diarrhea
  • Burning or pain with urination
  • Pain or discomfort in any part of your body
  • Contacts with sick people at home, work or school
  • The types of food you’ve eaten recently
  • Any recent exposures you have had to animals, including pets
  • Whether you have any implanted prosthetic or mechanical device, such as an artificial joint or mechanical heart valve
  • Any recent surgery, cuts or large areas of broken skin
  • Chronic inflammatory conditions such as rheumatoid arthritis
  • The types of medications you take
  • Recent immunizations
  • Recent travel, especially to a foreign country

Your doctor will take your temperature, either orally, in the ear canal or rectally. Depending on the suspected site of infection, your doctor may then examine your:

  • Skin for signs of rash or infection
  • Lymph nodes for swelling (a sign of nearby infection)
  • Eyes for redness or jaundice (yellowing of the whites of the eyes)
  • Mouth and throat for signs of pharyngitis (throat infection) or a Tooth abscess
  • Cardiovascular system for heart failure or infections involving the heart
  • Chest for lung infection
  • Abdomen for infections of the gallbladder, intestine or appendix
  • Joints for arthritis
  • Genitals for sexually transmitted diseases
  • Nervous system, for encephalitis (brain infection) or meningitis (inflammation/infection involving the membranes covering the brain)

Depending on your symptoms and the results of the physical examination, diagnostic tests may be necessary, including laboratory examination of body fluids (blood, urine, stool or spinal fluid); specialized X-rays or scans; or a biopsy (taking a sample of body tissue for laboratory examination).

Expected Duration

Depending on its cause, a fever may disappear in a day or persist for weeks.


You can reduce your risk of getting illnesses that cause fever by adopting these healthy habits:

  • Practice good hygiene by washing your hands frequently with soap and warm water, especially after using the toilet and before eating.
  • Limit exposure of infants to crowds and to people with known infections.
  • Cook and store foods properly to avoid food poisoning.
  • Keep a record of your current immunizations. Review this record with your doctor every year to confirm that your immunizations are up-to-date.
  • Contact your doctor before traveling to a foreign country to receive any recommended immunizations before your trip.


For adults with mild to moderate fevers below 102 degrees Fahrenheit, doctors often advise patients to:

  • Drink plenty of water and fruit juices to prevent dehydration (abnormally low levels of body water). Fluids act as internal coolants, and they also replenish vital salts and minerals (electrolytes), which may be lost during vomiting or diarrhea.
  • Temporarily eat light foods that are easy to digest.
  • Get plenty of rest.
  • To reduce fever, take ibuprofen (Advil, Motrin or others), acetaminophen (Tylenol) or aspirin according to label directions. Fevers may help to fight infection, so there is a controversy about whether fevers should be treated routinely or should be treated only if they are particularly severe.

Aspirin should not be given to infants and children younger than age 16 because of the risk of Reye’s syndrome, a life-threatening neurological disorder that can develop when children take aspirin during a viral illness. In children, reduce fever by using acetaminophen or ibuprofen, together with lukewarm sponge baths. If your child has a febrile seizure, call your doctor immediately.

If your doctor suspects that a bacterial infection is causing your fever, he or she will prescribe antibiotics.

When To Call A Professional

Contact your doctor if a fever exceeds 102 degrees Fahrenheit in an adult or 101 degrees Fahrenheit in an infant, or if a milder fever has not gone away within 24 hours. Seek immediate medical attention for any fever of 104 degrees Fahrenheit or above or for any fever accompanied by any of the following:

  • Convulsion
  • Loss of consciousness
  • Confusion
  • Stiff neck
  • Difficulty breathing
  • Severe pain anywhere in the body (especially the head, chest or abdomen)
  • Swelling or inflammation of any part of the body
  • Vaginal discharge that is discolored or foul-smelling
  • Urinary-tract symptoms (pain on urination, foul-smelling urine)

Also, any fever that occurs in infants younger than 3 months old should be brought to the immediate attention of the child’s doctor. Seek immediate medical help if a child with fever has a convulsion.


In most fever episodes due to common infections, the patient either recovers spontaneously or the doctor is able to identify and treat the underlying cause.

Brain damage may occur in people who have sustained body temperatures of 106 degrees Fahrenheit or above.

Johns Hopkins patient information

Last revised:

Diseases and Conditions Center

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.