Why do we get a fever when we have the flu?

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The raised body temperature not only kills some types of microbes, but also increases the count of the white blood cells that destroy infected cells.

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Fever, also known as pyrexia, is defined by increased body temperature and occurs in response to sickness, including the flu.

Generally, body temperature over 38 Celsius or 100.4 Fahrenheit is considered a fever. Common symptoms of fever include sweating, lethargy, dehydration and, paradoxically, chills.

Brain's limbic system
Brain’s limbic system

Fever plays an important part in helping the body fight off pathogens. Because normal body temperature can vary between individuals, the point at which fever is present can also be different for people.

Involvement of the Hypothalamus

The hypothalamus, the part of the brain that regulates body temperature, is involved in the development of fever. When the hypothalamus is activated by sickness, it essentially turns up the thermostat in the body.

Epinephrine, a neurotransmitter that is also released upon consumption of caffeine, is used to increase the heat. Pyrogens, which are constantly circulating the body, travel to the hypothalamus and activate receptors in order to stimulate this process.

The Purpose of Fever

Although fever can cause unpleasant symptoms, it helps the body fight infections in a number of ways. Fever is also a good sign that the immune system is working effectively.

White Blood Cells
White Blood Cells

As the body temperature rises, a variety of immunological processes are increased. In fact, experiments performed on mice have found that heated mice had more white blood cells, which tag pathogens for attack, compared to unheated rodents.

Furthermore, certain bacteria and viruses are very sensitive to temperature changes and just the heat produced by the body during a fever can kill them.

In fact Hippocrates and other physicians have historically attempted to raise body temperature to help patients recover from illness.

Thermometer Readings Vary by Body Location and Time

Sick woman with flu
Sick woman with flu

Thermometers are used to detect fevers, but their readings may be higher or lower depending on the time and the part of the body where they are used. For example, the temperature in the ear may be different from the temperatures detected in the mouth or the armpit.

Furthermore, menstrual cycle can affect temperature in women of childbearing age, and temperature normally varies based on the time of day.

While body temperature is commonly lower in the morning, it can be a bit higher at night. These factors must be accounted for when determining whether fever is present.

Fever Should Not Necessarily Be Reduced

Ibuprofen or paracetamol
Ibuprofen or paracetamol

Antipyretics are drugs that can reduce fever, but their use can actually be harmful when fever is not directly threatening a sick person.

While anti-inflammatory drugs, such as ibuprofen, can lower body temperature and fight associated symptoms during fever, they can actually prolong or worsen the illness.

This tradeoff may be trivial during a mild case of the flu, but severe sickness may require fever for successful resolution. This does not mean that a fever should never be treated. If the body temperature is very high it can cause brain damage.


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