Fever is one of your body’s reactions to infection. What’s normal for you may be a little higher or lower than the average temperature of 98.6 F (37°C). But a rectal temperature higher than 100.4 F (38°C) is always considered a fever. A rectal temperature reading is generally 1 degree F (about 0.5°C) higher than an oral reading.

For very young children and infants, even slightly elevated temperatures may indicate a serious infection. In newborns, a subnormal temperature – rather than a fever – also may be a sign of serious illness.

Don’t treat fevers below 102 F (38.9°C) with any medications unless advised to do so by your doctor. If you have a fever of 102 F (38.9°C) or higher, your doctor may suggest taking an over-the-counter medication, such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin, others). Adults may also use aspirin. But don’t give aspirin to children. It may trigger a rare, but potentially fatal, disorder known as Reye’s syndrome. Also, don’t give ibuprofen to infants younger than 6 months of age.

Fahrenheit-Celsius conversion table
°F	°C
105	40.6
104	40.0
103	39.4
102	38.9
101	38.3
100	37.8
99	37.2
98	36.7
97	36.1
96	35.6

How to take a temperature
You can choose from several types of thermometers. Today most have digital readouts. Some take the temperature quickly from the ear canal and can be especially useful for young children and older adults. Other thermometers can be used rectally, orally or under the arm. If you use a digital thermometer, be sure to read the instructions so you know what the beeps mean and when to read the thermometer. Under normal circumstances, temperatures tend to be highest around 4 p.m. and lowest around 4 a.m. Because of the potential for mercury exposure or ingestion, glass mercury thermometers have been phased out and are no longer recommended.

Rectally (for infants)
To take your child’s temperature rectally

• Place a dab of petroleum jelly or other lubricant on the bulb.
• Lay your child on his or her stomach.
• Carefully insert the bulb one-half inch to one inch into the rectum.
• Hold the bulb and child still for three minutes. To avoid injury, don’t let go of the thermometer while it’s inside your child.
• Remove and read the temperature as recommended by the manufacturer.
• A rectal temperature reading is generally 1 degree F (about 0.5°C) higher than a simultaneously taken oral reading.

Taking a rectal temperature is also an option for older adults when taking an oral temperature is not possible.

Orally
To take your temperature orally

• Place the bulb under your tongue.
• Close your mouth for the recommended amount of time, usually three minutes.

Under the arm (axillary)
Although it’s not the most accurate way to take a temperature, you can also use an oral thermometer for an armpit reading

• Place the thermometer under your arm with your arm down.
• Hold your arms across your chest.
• Wait five minutes or as recommended by your thermometer’s manufacturer. Then remove the thermometer and read the temperature.
• An axillary reading is generally 1 degree F (about 0.5°C) less than an oral reading.

To take your child’s axillary temperature, sit your child in your lap with your child facing to the side. Place the thermometer under your child’s near arm, which should be against your chest.

Get medical help for a fever in these cases

• If a baby is younger than 3 months of age and has a rectal temperature of 100.4 F (38°C) or higher. Even if your baby doesn’t have other signs or symptoms, call your doctor just to be safe.
• If a baby is older than 3 months of age and has a temperature of 102 F (38.9°C) or higher.
• If a newborn has a lower than normal temperature – less than 97 F (36.1°C) rectally.
• If a child younger than age 2 has a fever for more than one day, or a child age 2 or older has a fever for more than three days. If your child has a fever after being left in a very hot car, seek medical care immediately.
• If an adult has a temperature of more than 103 F (39.4°C) or has had a fever for more than three days.

Call your doctor immediately if any of these signs or symptoms accompanies a fever

• A severe headache.
• Severe swelling of the throat.
• Unusual skin rash.
• Unusual eye sensitivity to bright light.
• A stiff neck and pain when the head is bent forward.
• Mental confusion.
• Persistent vomiting.
• Difficulty breathing or chest pain.
• Extreme listlessness or irritability.
• Abdominal pain or pain when urinating.
• Any other unexplained symptoms.

When reporting a fever to your doctor, don’t attempt to convert from a rectal reading to an oral reading. It’s simpler to just report what the reading was and how you took it.

Source: www.mayoclinic.com