Jennifer Baker
Product Manager, Anatomy & Physiology

March 2017

A healthy human body has an average temperature of 98.6 degrees Fahrenheit, and a fluctuation of 1 degree above or below that mark is considered normal. Human bodies maintain this range through a process called homeostasis, despite many factors that can influence temperature (such as the temperature of the environment). Body temperature is most accurately measured rectally, but is often measured orally under the tongue, or as an axillary (armpit) temperature. Sometimes the body will increase its internal temperature above the normal range in an effort to rid the body of pathogens—unwelcome bacteria, viruses, or parasites. This response is commonly called a fever.

How our bodies fight infection

When a person is exposed to a pathogen, the body elicits an immune response. The response is often in the form of an increase in temperature. A localized reaction, one that takes place only in the area of exposure, such as a scrape or bruise, can feel hot to the touch. A systemic reaction, one that takes place across many or all of the parts of the body, can result in an increase of temperature throughout the entire body. A fever is the body’s attempt to create an environment that is not welcoming to the pathogen. Bacteria, viruses, and parasites that infect humans thrive within the normal body temperature range, and a notable increase in body temperature makes it harder for them to grow and reproduce successfully.

Pyrogens trigger the body’s fever response. A pyrogen can be endogenous, which means they are produced by the body, or exogenous, which means they are produced by a pathogen, typically a bacterium. Pyrogens make their way to the bloodstream and are carried to the hypothalamus, a gland located at the base of the brain. The hypothalamus acts as a thermostat in the body. In response to the presence of pyrogens, the hypothalamus signals the body to increase its overall temperature, creating an inhospitable environment for pathogens.

When to seek help

In most cases, a short-term increase in overall body temperature allows the body to rid itself of infection. A fever can be prolonged or reach a temperature that is too high. In these instances, proteins required for normal body functions can be compromised; high temperatures can cause the proteins to unravel and become unusable. In adults, a fever that exceeds 103 degrees Fahrenheit or that lasts longer than 3 days requires medical attention.