A fever may occur as a reaction to:
- Infection. This is the most common cause of a fever. Infections may affect the whole body or one body part.
- Medicines. These include antibiotics, narcotics, antihistamines, and many others. This is called a "drug fever." Medicines like antibiotics raise the body temperature directly. Other medicines keep the body from resetting its temperature when other things cause the temperature to rise.
- Severe trauma or injury. This may include heart attack, stroke, heatstroke, or burns.
- Other medical conditions. These include arthritis, hyperthyroidism, and even some cancers, such as leukemia and lung cancer.
Low body temperature (hypothermia)
A very low body temperature (hypothermia) can be serious or even deadly. Low body temperature usually happens from being out in cold weather. But it may also be caused by alcohol or drug use, going into shock, or certain disorders such as diabetes or low thyroid.
A low body temperature may occur with an infection. This is most common in newborns, older adults, or people who are frail. A very bad infection, such as sepsis, may also cause an abnormal low body temperature.
Heatstroke occurs when the body fails to control its own temperature and body temperature keeps rising. Symptoms of heatstroke include mental changes (such as confusion, delirium, or unconsciousness) and skin that is red, hot, and dry, even under the armpits.
Heatstroke can be deadly. It needs emergency medical treatment. It causes severe dehydration and can cause body organs to stop working.
There are two types of heatstroke.
Classic heatstroke can happen even when a person isn't doing much, as long as it's hot and the body isn't able cool itself well enough by sweating. The person may even stop sweating. Classic heatstroke may develop over several days. Babies, older adults, and people who have chronic health problems have the greatest risk of this type of heatstroke.
Exertional heatstroke may happen when a person is working or exercising in a hot place. The person may sweat a lot, but the body still makes more heat than it can lose. This causes temperature to rise to high levels.