Temperature is a physical quantity that expresses the attribute of hotness or coldness of a substance. It is measured with a thermometer and reflects the kinetic energy of the vibrating and colliding atoms making up the substance. The temperature of a body in a state of thermodynamic equilibrium is always positive relative to the absolute zero. The Kelvin scale is widely used in science and technology to measure temperature, and it is based on the absolute zero.

For humans, the average body temperature has traditionally been defined as 98.6°F (37°C) . However, recent studies suggest that the baseline for most people is closer to 98.2°F (36.8°C) . Body temperatures vary slightly from person to person and at different times of day. A temperature taken using a mouth thermometer (oral temperature) thats 100°F (37.8°C) or higher is generally considered to be a fever. Infants and young children normally have slightly higher body temperatures than older children and adults, so infants and younger children have slightly higher fever temperatures.

When a body temperature rises a few degrees above normal, it can be a sign that the immune system has been activated, often to fight an infection. A fever itself isn’t an illness, but rather a symptom of a wide range of health conditions. The medical community generally defines a fever as a body temperature above 100.4°F (38°C) . A body temperature between 100.4°F and 102.2°F (38°C and 39°C) is usually considered a low-grade fever. When body temperature rises above 102.2°F (39°C), it can be a sign of a more serious infection.

To take a temperature, several types of thermometers can be used, including oral, rectal, ear (tympanic), and forehead (temporal artery) thermometers. Oral and rectal thermometers generally provide the most accurate measurement of core body temperature. Ear or forehead thermometers, although convenient, provide less accurate temperature measurements.