Normal body temperature is a moving target:
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In children younger than six months of age, the daily variation is small.
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In children 6 months to 2 years old, the daily variation is about 1 degree.
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Daily variations gradually increase to 2 degrees per day by age six.
Body temperature varies less in adults. However, a woman's menstrual cycle can elevate temperature by one degree or more.
Your body temperature is usually highest in the evening. It can be raised by physical activity, strong emotion, eating, heavy clothing, medications, high room temperature, and high humidity. This is especially true in children.
A rectal temperature up to 100.4F (38C) may be entirely normal (no fever). A rectal temperature of 100.5F or above should always be considered a fever. Lower values might be a fever, depending on the person.
Fever is not an illness. Far from being an enemy, it is an important part of the body's defense against infection. Many infants and children develop high fevers with minor viral illnesses. While a fever signals to us that a battle might be going on in the body, the fever is fighting for the person, not against.
Most bacteria and viruses that cause infections in humans thrive best at 98.6F. Raising the temperature a few degrees can give the body the winning edge. In addition, a fever activates the body's immune system to make more white blood cells, antibodies, and other infection-fighting agents.
Many parents fear that fevers will cause brain damage. Brain damage from a fever generally will not occur unless the fever is over 107.6F (42C). Many parents also fear that untreated fevers will keep going higher and higher, up to 107F or even more. Untreated fevers caused by infection will seldom go over 105F unless the child is overdressed, or trapped in a hot place. The brain's thermostat will stop the fever from climbing above 106F.
Some parents fear that fevers will cause seizures. For the great majority of children, this is not the case. However, febrile seizures do occur in some children. Once a child is already known to have a high fever, a febrile seizure is unlikely with the current illness. In any event, simple febrile seizures are over in moments with no lasting consequences.
Although common infections are the most common causes of elevated body temperature, fevers have a long list of other causes, including toxins, cancers, and autoimmune diseases.
Heatstroke is a particularly dangerous type of high temperature, because the body is not able to stop the temperature from continuing to rise. It can happen when a child is left in a hot car, or when a football player exercises too strenuously, without enough to drink.
Unexplained fevers that continue for days or weeks are referred to by doctors as fevers of undetermined origin (FUO). Most are eventually found to be caused by a hidden infection.
