A high fever can accompany viruses, infections, or reactions to immunizations. It is the body’s way of attempting to rid itself of infection, and it can be effective. However, most people are curious to know when the fever begins to be dangerous, and in most cases, it isn’t that dangerous on its own. Very high fever can be dangerous for those with extremely suppressed immune systems, and for very young children. In general, a slightly elevated body temperature isn’t of great concern, unless it lasts for a lengthy period of time.

There are guidelines for determining what is considered a high fever and when to call a doctor about presence of fever. These vary depending upon age. It should first be understood what normal body temperature is. This is typically 98.6 degrees F (37 degrees C), as taken orally, rectally or with ear thermometers. Readings under the arm, or axillary temperatures, are usually about one degree F lower, or about 36.5 degrees C.

In infants three months or younger, any fever is cause for concern and any rectal reading above 100.4 degrees F (38 C) is indication to call a doctor. This is because infants may quickly succumb to illness and they can dehydrate in a hurry. It is very dangerous to ignore evidence of fever in an infant, and people might look for fever if they note extra irritability, sleepiness, or if a baby seems very warm. It’s also important to notify doctors if an infant’s temperature drops below 97 degrees F (36.11 C).

Once a child is over three months old, a high fever is considered 102 degrees F (38.88 C). This should always mean that a doctor should be called to determine if the child should be seen. A lower fever could be an emergency condition if a child is not responding and is very sleepy. This could signal the child has a condition like meningitis, and the child should be taken to an emergency room right away.

Children under 5 are also more vulnerable to a type of seizure caused by fever. These are called febrile seizures. Most of these will not permanently harm a child, but if one occurs, it is a medical emergency and a child should see a doctor immediately. Febrile seizures are not particularly common in older kids or in adults.

In adults, a high fever is usually considered 103 degrees F (39.44 degrees C). When such a fever occurs, particularly if it doesn’t go down within a day, contact a doctor. Also, any person of any age who sustains a fever for several days, even if it’s lower than the high fever range, could have an infection and should contact a physician.