You have a fever when your temperature rises above its normal range. What's normal for you may be a little higher or lower than the average normal temperature of 98.6 F (37 C).

Depending on what's causing your fever, additional fever signs and symptoms may include:

• Sweating
• Chills and shivering
• Headache
• Muscle aches
• Loss of appetite
• Irritability
• Dehydration
• General weakness

Children between the ages of 6 months and 5 years might experience febrile seizures. About a third of the children who have one febrile seizure will have another one, most commonly within the next 12 months.

Infants

An unexplained fever is greater cause for concern in infants and in children than in adults. Call your baby's doctor if your child is:

• Younger than age 3 months and has a rectal temperature of 100.4 F (38 C) or higher.
• Between ages 3 and 6 months and has a rectal temperature up to 102 F (38.9 C) and seems unusually irritable, lethargic or uncomfortable or has a temperature higher than 102 F (38.9 C).
• Between ages 6 and 24 months and has a rectal temperature higher than 102 F (38.9 C) that lasts longer than one day but shows no other symptoms. If your child also has other signs and symptoms, such as a cold, cough or diarrhea, you might call your child's doctor sooner based on severity.

Children

There's probably no cause for alarm if your child has a fever but is responsive — making eye contact with you and responding to your facial expressions and to your voice — and is drinking fluids and playing.

Call your child's doctor if your child:

• Is listless or irritable, vomits repeatedly, has a severe headache or stomachache, or has any other symptoms causing significant discomfort.
• Has a fever after being left in a hot car. Seek medical care immediately.
• Has a fever that lasts longer than three days.
• Appears listless and has poor eye contact with you.

Ask your child's doctor for guidance in special circumstances, such as a child with immune system problems or with a pre-existing illness.

Adults

Call your doctor if your temperature is 103 F (39.4 C) or higher. Seek immediate medical attention if any of these signs or symptoms accompanies a fever:

• Severe headache
• Unusual skin rash, especially if the rash rapidly worsens
• Unusual sensitivity to bright light
• Stiff neck and pain when you bend your head forward
• Mental confusion
• Persistent vomiting
• Difficulty breathing or chest pain
• Abdominal pain or pain when urinating
• Convulsions or seizures

Fever occurs when an area in your brain called the hypothalamus (hi-poe-THAL-uh-muhs) — also known as your body's "thermostat" — shifts the set point of your normal body temperature upward. When this happens, you may feel chilled and add layers of clothing or wrap up in a blanket, or you may shiver to generate more body heat, eventually resulting in an elevated body temperature.

Normal body temperature varies throughout the day — it's lower in the morning and higher in the late afternoon and evening. Although most people consider 98.6 F (37 C) normal, your body temperature can vary by a degree or more — from about 97 F (36.1 C) to 99 F (37.2 C) — and still be considered normal.

Fever or elevated body temperature might be caused by:

• A virus
• A bacterial infection
• Heat exhaustion
• Certain inflammatory conditions such as rheumatoid arthritis — inflammation of the lining of your joints (synovium)
• A malignant tumor
• Some medications, such as antibiotics and drugs used to treat high blood pressure or seizures
• Some immunizations, such as the diphtheria, tetanus and acellular pertussis (DTaP) or pneumococcal vaccine

Sometimes the cause of a fever can't be identified. If you have a fever for more than three weeks and your doctor isn't able to find the cause after extensive evaluation, the diagnosis may be fever of unknown origin.

Children between the ages of 6 months and 5 years may experience fever-induced convulsions (febrile seizures), which usually involve loss of consciousness and shaking of limbs on both sides of the body. Although alarming for parents, the vast majority of febrile seizures cause no lasting effects.

If a seizure occurs:

• Lay your child on his or her side or stomach on the floor or ground
• Remove any sharp objects that are near your child
• Loosen tight clothing
• Hold your child to prevent injury
• Don't place anything in your child's mouth or try to stop the seizure