A fever can be caused by a wide variety of things, including dehydration, overexertion, mosquito bites, bee stings, an allergic or toxic reaction, or a viral or bacterial infection. Fever of unknown origin (FUO) is a condition defined as an elevated temperature lasting for a week or more without an identifiable cause.

In most cases, a fever is the body's reaction to an acute viral or bacterial infection. It is not necessarily a dangerous condition. Rather, it is a sign that the body is defending itself against the infectious invader. Since viruses and bacteria do not survive as well in a body with an elevated temperature, fever is actually an ally in fighting infection. It is one of the ways in which the body defends and heals itself. An elevated temperature also increases the production of infection-fighting white blood cells and even increases their speed of response and enhances their killing capacity.

In an adult, the level of fever generally reflects the severity of the illness causing it. In a child, however, this is not necessarily the case. A child with a mild cold may have a 105°F fever, while a child with a serious illness-bacterial pneumonia, for example-may have only a 100°F fever.

In a newborn, the body's temperature control is not yet well developed. As a result, signs other than a fever-poor appetite, lethargy, and irritability-may be earlier indicators of an infection and therefore more helpful than temperature in assessing your newborn's condition. However, when a newborn has a persistently elevated (or low) temperature, he should be examined by a doctor.