What is Fever?

What is the definition of fever?

The question above seems to be simple, but many of us may not be clear about what fever exactly is? Fever (pyrexia in medical term) is known to all of us. But many of us may not have clear idea about what fever actually is or what the definition of fever is? We all know what fever is, i.e. rise of body temperature above that of normal body temperature. Many of us also know that normal body temperature is 98.4 degree Fahrenheit (98.4°F) or 37 degree Centigrade (37° C).

Studies on healthy individuals of 18-40 years of age reveal the normal body temperature differently. According to studies the mean oral temperature is 98.2° ± 0.7°F (or 98.2° ± 0.7° C), with lowest around 6 AM in the morning and highest at 4-6 PM. According to studies the maximum oral temperature around 6 AM is 37.2°C (or 98.9°F) and 37.7°C (99.9°F) between 4-6 PM. According to these studies a morning temperature of more than 37.2°C (or 98.9°F) and evening temperature of 37.7°C (99.9°F) can be defined as fever, which is true for 99th percentile of healthy individuals.

If rectal temperature is taken into account to define fever one should add 0.4°C (0.7°F) to the above definition of fever, because rectal temperature is generally 0.4°C (0.7°F) higher than oral temperature. The oral temperature is less than rectal temperature, because of heat loss during mouth breathing,

In general the normal temperature variation in a healthy individual is typically 0.5°C (0.9°F). But during recovery from fever/illness the temperature variation may be greater and as much as 1.0°C. During childhood the daily temperature variation is fixed, but for elderly individuals the daily temperature variation is reduced and they may not develop fever as a result of even severe infection.

For menstruating women the morning temperature is generally lower 2 weeks preceding ovulation and rises during ovulation to approximately 0.6°C (1°F), where it remains till next menstruation.

How human body regulates/controls body temperature?

Our body temperature is regulated/controlled by hypothalamus. Neurons of hypothalamus receive two types of signals, one from blood bathing different organ systems (including skin) and the other from peripheral nerves that transmit information to hypothalamus from warmth/cold receptors in the skin. These two types of signals are then interpreted by the thermoregulatory center present in the hypothalamus to maintain normal body temperature. In a healthy individual and in neutral environment the metabolic rate produces more heat/energy to maintain core body temperature of 37°C.

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