Fever, or pyrexia, is when the internal body temperature rises to levels that are considered above normal.
It normally indicates the present of another condition, usually an infection.
Average body temperature is about 98.6 °Fahrenheit (37 °Celsius), and temperatures above 100.4 °F (38 °C) are considered to be a fever. Three common sites for checking temperature are the mouth, under the arm, and the rectum.
Although unpleasant, a fever on its own is not generally considered dangerous. However, in young children, older adults, or individuals with other health concerns, a fever should be checked out by a doctor.
Fast facts on fever
Here are some key points about fever. More detail is in the main article.
Common symptoms of a fever include chills and shaking
Bacterial or viral infections are common causes.
A fever can be part of the body’s way to fight infection.
Aspirin and acetaminophen can help reduce fever, but aspirin is not suitable for children.
A fever is not usually a problem in itself, but a symptom of another condition. It indicates that there is something wrong with some part of the body.
It can happen when something goes wrong with one of a wide range of functions.
As one researcher explains, “The febrile response is orchestrated by the central nervous system through endocrine, neurological, immunological, and behavioral mechanisms.”
A fever can help the body fight infection, because it increases the amount of antiviral and anticancer interferon in the blood. This makes it difficult for bacteria and viruses to replicate.
Body temperature can also help measure the success of medical treatments.
Conditions that cause a fever
Infections are the most common cause of fever, but various conditions, illnesses, and medicines can raise the body temperature.
infections and infectious diseases, such as influenza, common cold, HIV, malaria, infectious mononucleosis, and gastroenteritis
legal and illegal drugs, including antibiotics, amphetamines, and cocaine
trauma or injury, such as a heart attack, stroke, heatstroke, heat exhaustion, or burns
damage to tissue, such as from hemolysis (breaking open of red blood cells to release hemoglobin), surgery, heart attack, crush syndrome, and hemorrhage
other medical conditions, such as skin inflammation, arthritis, hyperthyroidism, some cancers, lupus, inflammatory bowel disease, blood clots, metabolic disorder, gout, and embolisms
Antibiotics, narcotics, barbiturates, and antihistamines can cause “drug fevers” due to adverse reactions, withdrawal, or because of the design of the drug.
How does the body regulate temperature?
A person’s temperature is determined by the body’s thermoregulatory set-point.
The body increases this set-point in response to threats, such as bacterial or viral infections. When the set-point rises, a fever occurs. At this point, the body believes it has hypothermia. It perceives that it is too cold.
As the body works towards meeting the new temperature set-point, symptoms commonly associated with a fever emerge, such as feelings of being cold, increased heart rate, stiffness, and shivering.
Body temperature is controlled by a part of the brain called the hypothalamus, and a network of systems also play a key role. These form a circuit that regulates the temperature. The hypothalamus sends signals to the body telling it to warm up or cool down.
Fevers can be classified in different ways.
One way is the length of time. A fever can be:
acute, lasting less than 7 days, as in a viral upper respiratory tract infection
sub-acute, lasting up to 14 days, as, for example, in typhoid
chronic or persistent, lasting over 14 days, as in tuberculosis, HIV, and cancers
They can also be classed according to severity:
The height of the temperature may help indicate what type of problem is causing it.
Fevers can also be:
sustained or continuous, where it does not fluctuate more than 1.5 °F (1 °C) over 24 hours,
but is never normal in this time
intermittent, when the fever occurs for several hours in the day, but not all the time
remittent, when it fluctuates by more than 2 °C but does not become normal
Typhoid may underlie a sustained fever, tuberculosis tends to cause an intermittent fever, and infective endocarditis may trigger a remittent fever.
Fevers that exist for days or weeks with no explanation are called fevers of undetermined origin (FUO).
A fever is a symptom, but it can occur with its own symptoms and with other symptoms.