A fever is a higher-than-normal body temperature, which is not an illness in itself, but a symptom that often occurs when the body is fighting an infection.
The standard of 98.6° F (37° C) as a normal average oral temperature was established in the 1800s but recent studies have shown the average body temperature has dropped to 97.5° F (36.4° C). It is believed this is due to a combination of more accurate thermometers, lower rates of infection, and lower metabolic rates because people weigh more on average today than they did in the 1800s.
Normal temperatures can vary throughout the day, with lower temperatures occurring in the early morning and higher temperatures in the late afternoon, sometimes reaching 99.9° F (37.7° C) in adults.
Technically, 99.9° F would be considered a fever only in babies under one year and only when measured axially (in the armpit), but not in adults. A core (rectal) body temperature of 100.4° F (38.0° C) or higher in adults, and 99° F (37.2° C) (armpit) or 100.4° F (38° C) (rectal) in babies under one year is considered a fever.
A fever isn't usually a concern unless it reaches 103° F (39.4° C) or higher.
Contact your doctor if you have a fever that:
- Is 103° F (39.4° C) or higher
- Does not respond to fever-reducing medicines, such as ibuprofen (Advil, Motrin) or acetaminophen (Tylenol)
- Lasts more than 3 days
Seek medical attention right away if you have symptoms that accompany a fever such as:
- Severe headache
- Unusual sensitivity to light
- Unusual skin rash, especially if the rash worsens or spreads quickly
- Confusion
- Persistent vomiting
- Shortness of breath or chest pain
- Neck pain and stiffness when looking downwards
- Severe abdominal pain
- Nausea or vomiting
- Pain or burning when urinating
- Seizures
What Causes Fever?
Fever may be caused by:
- Infections, such as with viruses or bacteria (most common cause)
- Certain inflammatory conditions such as rheumatoid arthritis
- Heat exhaustion or heatstroke
- Cancer
- Certain medications, such as high blood pressure medications, antibiotics, anticholinergics, and seizure medications
- Some vaccines, including the diphtheria, tetanus and acellular pertussis (DTaP), pneumococcal vaccine, and measles vaccine
- Overdressing
- This is more common in newborns and infants because they are unable to regulate their body temperature as well as older children and adults
What Is the Treatment for Fever?
The goal of treatment for a mild fever is to relieve symptoms, and may include:
- Drinking fluids to stay hydrated
- Resting
- Use of over-the-counter (OTC) fever reducers such as acetaminophen (Tylenol or ibuprofen (Advil, Motrin)
- Do not give aspirin to children because it may trigger Reye's syndrome, a rare but potentially life-threatening disorder
- Antibiotics
- For fevers caused by bacterial infections
- Antibiotics are not used to treat infections caused by viruses
Infants may need to be admitted to a hospital for treatment for a fever because it may be a sign of a serious infection that requires intravenous (IV) medications.
From 
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https://www.merckmanuals.com/professional/pediatrics/symptoms-in-infants-and-children/fever-in-infants-and-children
https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Fact-Sheets/Febrile-Seizures-Fact-Sheet
https://www.merckmanuals.com/home/infections/biology-of-infectious-disease/fever-in-adults#:~:text=Fever%20is%20an%20elevated%20body,by%20a%20rectal%20thermometer.