A fever is defined as a body temperature of 38°C (100.4°F) or higher. Normal body temperature is usually 37°C (98.6°F), although it can be about a half degree Celsius higher or lower for some people and that’s normal for them.

A fever occurs when your brain sets your body temperature higher than normal. Normal body temperature can change during the day based on activity level, hormone levels and whether you are sleeping or awake.

Your normal body temperature is regulated or set by an area of your brain called your hypothalamus. Fever occurs when your body’s defense system — your immune system — reacts to an infection or illness, triggering your brain to raise your body temperature. This helps your immune system fight off the illness or infection, but a very high temperature can be dangerous.

Fever is not the same as hyperthermia, which means your body temperature goes up due to the outside temperature. Hyperthermia is not caused by your brain and immune system. An example would be heat stroke.

Fever grades in adults

In an adult, a fever may be elevated, high or very high:

• An elevated body temperature is between 37.5°C and 38°C (99.5°F to 100.4°F).
• A fever is 38°C to 39.5°C (100.4°F to 103.1°F).
• A high fever is 39.5°C (103.1°F).
• A very high fever is above 41°C (105.8°F ).

Fever in babies and children

According to the American Academy of Pediatrics (AAP), normal temperature in children varies with a child’s age, time of day and level of activity.

Fever is defined as a temperature of 38°C (100.4°F).

• A fever that requires a call to the pediatrician right away would be 38°C (100.4°F) or higher for a baby under 3 months old.
• A fever above 40°C (104°F) requires a call to the doctor at any age.

How to measure fever

To take your temperature, a digital thermometer is recommended instead of a mercury thermometer, especially in children, because mercury from a broken thermometer may be toxic.
There are three types of digital thermometers. There are many variations, so it is important to read and follow the thermometer instructions carefully. They all must be turned on and they will beep to tell you when to read them:

• A digital multi-use thermometer can be used to take a rectal, oral (mouth) or axillary (armpit) temperature in adults and children. In general, oral and armpit temperatures have not shown to be as accurate as rectal. Oral use is not recommended for children under age 4, according to the AAP, and AAP notes that rectal use is best, especially for infants and children under 3 months. Always clean the thermometer before and after use, and do not use a thermometer orally that has been used rectally. Do not take an oral temperature for 30 minutes after a hot or cold drink.
• A temporal or forehead artery thermometer reads your temperature from a blood vessel under the skin of your temple or forehead. You will need to check the instructions to find out how far away from your skin to hold it. If you have been in hot or cold temperatures, you should adjust to normal temperature before taking a reading.
• A tympanic digital thermometer measures heat inside your ear canal. This type can be used for anyone except babies.

When and how to treat a fever

Since ancient times, fevers have been associated with illness. It was not until the invention of the thermometer that normal temperature and fever temperatures were defined. Over time, the definitions have gone up and down slightly.

At this time, there is a debate on when to treat a fever.

• Some experts say a fever is a sign that your immune system is working and anything other than a very high fever should be left to run its course.
• Others fear that fever may be damaging to your body and should be treated. Very high fever may damage body cells, especially brain cells.

There are no guidelines for when to treat a fever. Treatment may depend on the cause and other symptoms, but most experts agree that a very high fever should be treated.

• The usual treatment for adults is an over-the-counter medicine such as acetaminophen, ibuprofen or aspirin. Rest and drinking lots of fluid are also recommended because fever can lead to fluid loss and dehydration.
• The AAP recommends acetaminophen or ibuprofen, but not aspirin for treating fever in children. (Aspirin in children can raise the risk of a harmful condition called Reye’s syndrome.)
  • For children over age 2, follow the label directions according to age or weight to give the right dose.
  • For children under age 2, call your pharmacist or pediatrician to find the correct dose.
  • Children should also drink fluids to avoid dehydration.
  • Using ice packs or alcohol rubs to reduce fever is not recommended.

When to call your doctor for fever

For children, call the doctor if:

• A child less than 3 months old has a fever over 38°C (100.4°F). Always call for any child with a fever 40°C (104°F) or higher.
• A fever occurs with other symptoms, including headache, stiff neck, trouble breathing, rash, severe sore throat or ear pain, or persistent vomiting
• A fever lasts more than 24 hours in a child under 2 years old
• A fever causes a seizure
• A fever lasts more than 3 days in a child over 2 years old

For adults and older children, call if a fever is 104°F (40°C) or higher or if there are symptoms that include seizure, stiff neck, confusion, trouble breathing or any severe pain.