Fever is a temporary increase in body temperature, often due to an illness or sickness. Having a fever is a sign that something out of the ordinary is going on in your body. Fever is one of the body’s most effective ways of fighting infection.
Normal body temperatures can vary and are influenced by factors such as exercise, eating, sleeping and the time of the day – the lowest temperature is usually recorded at around 3 am, the highest at 6 pm.
The average normal body temperature taken in the mouth is 37ºC (98.6ºF), but anywhere between 36.5ºC and 37.2ºC (97.7ºF and 99ºF) may be normal. Normal armpit temperatures are 0.2ºC to 0.3ºC lower than this.
A temperature of 38ºC (100.4ºF) or above is usually considered to be a significant fever – you should measure it again after two to three hours.
For an adult, a fever may be uncomfortable, but usually isn’t a cause for concern unless it reaches 103 F (39.4 C) or higher. For infants and toddlers, a slightly elevated temperature may indicate a serious infection.
Fevers generally go away within a few days. A number of over-the-counter medications lower a fever, but sometimes it’s better left untreated. Fever seems to play a key role in helping your body fight off a number of infections.
Fever occurs when an area in your brain called the hypothalamus, 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 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.
A fever 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 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 a fever of unknown origin.
The classification of fever depends on how long they last. Whether they are of higher temperatures or how long they take before going. The following are the classification of fever:
A fever can be classified according to how severe it is. The following are the indications of a severe fever:
- Low grade, from 100.5–102.1°F or 38.1–39°C
- Moderate, from 102.2–104.0°F or 39.1–40°C
- High, from 104.1–106.0°F to or 40.1-41.1°C
- Hyperpyrexia, above 106.0°F or 41.1°C
The height of the temperature may help indicate what type of problem is causing it.
Length of time
A fever can be:
- Acute if it lasts less than 7 days
- Sub-acute, if it lasts up to 14 days
- Chronic or persistent, if it persists for over 14 days
- Fevers that exist for days or weeks with no explanation are called fevers of undetermined origin (FUO).
One gets a fever when the body 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:
- Chills and shivering
- Muscle aches
- Loss of appetite
- 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.
To check your/your child’s temperature, you can choose from several types of thermometers, including oral, rectal, ear (tympanic) and forehead (temporal artery) thermometers.
Although it’s not the most accurate way to take a temperature, you can use an oral thermometer for an armpit (axillary) reading:
- Place the thermometer in the armpit and cross your arms or your child’s arms over the chest.
- Wait four to five minutes. The axillary temperature is slightly lower than an oral temperature.
- If you call your doctor, report the actual number on the thermometer and where on the body you took the temperature.
Use a rectal thermometer for infants:
- Place a dab of petroleum jelly on the bulb.
- Lay your baby on his or her tummy.
- Carefully insert the bulb 1/2 to 1 inch (1.3 to 2.5 centimeters) into your baby’s rectum.
- Hold the bulb and your baby still for three minutes.
- Don’t let go of the thermometer while it’s inside your baby. If your baby squirms, the thermometer could go deeper and cause an injury.
Fever Symptoms In Infants
Unexplained fever is a 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.
Fever Symptoms In 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 a 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 the child’s doctor for guidance in special circumstances, such as a child with immune system problems or with a pre-existing illness.
Fever Symptoms In Adults
- 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
When to contact a doctor
Fever is rarely harmful, but at very high temperatures it can cause problems. For example, some small children with a high fever will have a seizure, known as a febrile convulsion.
A high fever may also be a sign of serious illness. Call your doctor if you’re at all worried, especially if:
- You suspect an infection, which might need antibiotic treatment
- It’s in a very young child
- The child becomes unusually sleepy or doesn’t respond
- The fever is higher than 38.5ºC (101.3ºF)
- The child won’t take fluids
- A child or adult has a stiff neck, an unusual rash or difficulty breathing
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
Most seizures stop on their own. Take your child to the doctor as soon as possible after the seizure to determine the cause of the fever.
To evaluate a fever, a doctor may:
- Ask questions about your symptoms and medical history
- Perform a physical exam
- Order tests, such as blood tests or a chest X-ray, as needed, based on your medical history and physical exam.
Because a fever can indicate a serious illness in a young infant, especially one 28 days or younger, your baby might be admitted to the hospital for testing and treatment.
For a low-grade fever, your doctor may not recommend treatment to lower your body temperature. These minor fevers may even be helpful in reducing the number of microbes causing your illness.
Treatment of infants
In infants, especially those younger than 28 days, your baby might need to be admitted to the hospital for testing and treatment. In babies this young, a fever could indicate a serious infection that requires intravenous (IV) medications and round-the-clock monitoring.
In the case of a high fever, or a low fever that’s causing discomfort, your doctor may recommend an over-the-counter medication, such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin IB, others).
Use these medications according to the label instructions or as recommended by your doctor. Be careful to avoid taking too much. High doses or long-term use of acetaminophen or ibuprofen may cause liver or kidney damage, and acute overdoses can be fatal. If your child’s fever remains high after a dose, don’t give more medication; call your doctor instead.
Don’t give aspirin to children, because it may trigger a rare, but potentially fatal, a disorder known as Reye’s syndrome.
Depending on the cause of your fever, your doctor may prescribe an antibiotic, especially if the doctor suspects a bacterial infection, such as pneumonia or strep throat.
Antibiotics don’t treat viral infections, but there are a few antiviral drugs used to treat certain viral infections. However, the best treatment for most minor illnesses caused by viruses is often rest and plenty of fluids.
Fever is part of your body’s defense against infection-causing germs. By itself, fever is usually harmless, though a high fever can be miserable. These steps may help you feel better if you develop the above-discussed symptoms:
- Drink plenty of fluids to help cool your body and prevent dehydration.
- Eat light foods that are easy to digest.
- Get plenty of rest.
- Take ibuprofen (Advil, Motrin, or others), naproxen, (Aleve, Naprosyn, or others), acetaminophen (Tylenol, others) or aspirin to help relieve head and body aches and lower your temperature.
- Take a slightly warm, not cool, bath or apply damp washcloths to the forehead and wrists.
- Dress lightly (even if you have chills).