Causes of Fevers

A fever is a natural part of your body's defense against illness. Infections are the most common cause of fever, and your body temperature may increase due to other causes, including medication use, inflammation, and other factors. In some cases, a fever can occur without a known cause.

Fevers aren't generally dangerous, and an increased temperature isn't even officially considered a fever until it is higher than 100.3 F degrees F.

This article describes the possible causes of a fever so you can get a better sense of the wide range of reasons why your body may react in this way—and when medical attention is necessary.

How Fevers Happen

A fever is caused by a physiological process that raises body temperature. This process is mediated by inflammatory cells, chemicals, hormones, and brain activity.

Some infectious organisms contain pyrogens, and many of the body's immune cells also contain pyrogens. These are chemical substances that induce a fever.

Pyrogens cause fever through a cascade of events:

• They travel to a region in the hypothalamus (in your brain) called the organum vasculosum lamina terminalis.
• This promotes the production of prostaglandins.
• Prostaglandins raise the body temperature through inflammation and vasoconstriction (narrowing of the blood vessels to prevent heat loss).

Heat can damage or kill temperature-sensitive pathogens, like viruses (i.e., rhinovirus) and bacteria (such as Streptococcus) that can make you sick. A fever is one of the tools your immune system uses as a weapon against infectious illnesses.

Besides infections, other situations can induce a fever by activating these physiological processes. This can include processes that induce inflammation or processes that directly affect the hypothalamus.

Infections

An infection caused by a virus, bacterium, or fungus can make you sick and lead to a fever. These illnesses are varied and can include the flu, strep throat, Lyme disease, kidney infections, ear infections, appendicitis, and more.

When your immune system recognizes a pathogen as an invader, it may release pyrogens into your blood. The pyrogens travel to the hypothalamus, which sits at the base of your brain and controls your temperature. Prostaglandins send the message that your temperature needs to be raised, which can make it impossible for certain pathogens to survive.

Immunizations

Vaccines can sometimes trigger a mild fever. That's because they introduce disabled pathogens or particles resembling a pathogen into your body so your immune system can learn to recognize and fight them if you are exposed at some time in the future.

The fever is a sign that the vaccine triggered an immune response (including pyrogens, prostaglandins, and action by the hypothalamus)—just as it was designed to do.

Inflammation and Related Diseases

Inflammation occurs as part of the body's immune response to infections, and it is also part of the healing process of many illnesses. Some of the chemicals produced by the inflammatory process are pyrogens, so the inflammatory process itself can kick off the chain of events that leads to an increase in body temperature.

Autoimmune and inflammatory diseases are associated with fevers that may come and go as the disease flares and remits.

Autoimmune diseases that are associated with fevers include:

• Rheumatoid arthritis
• Lupus
• Multiple sclerosis

Autoinflammatory diseases linked to fevers include:

• Familial Mediterranean fever
• Adult-onset Still's disease

Fevers are common in some types of cancers, especially blood cancers such as lymphoma and leukemia. While the cause of this isn't well understood, there are several contributing factors. Some types of cancer cause inflammation, and some cancer cells may produce pyrogenic substances.

Other diseases that are inflammatory and may involve pyrogens include:

• Liver diseases
• Atherosclerosis
• Obesity

Traumatic brain injury can cause fever if the hypothalamus is affected or if messages to the hypothalamus are disrupted.

Sometimes, blood clots are associated with fever. Surgery can also lead to inflammation and inflammation-related fevers.

Medications, Illegal Drugs, and Alcohol

Certain drugs and medications can trigger a fever.

Serotonin syndrome is a serious reaction that includes many symptoms, including a fever. It can occur with very high doses or combinations of monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), and certain antipsychotics.

Alcohol withdrawal can lead to imbalances in brain chemistry and, in the most severe cases, delirium tremens (DTs). Fever is just one of many symptoms of DTs.

Misuse of drugs in the amphetamine class—including the illegal drugs methamphetamine, ecstasy, and bath salts (synthetic drugs that produce effects similar to cocaine)—can also increase body temperature.

Fever of Unknown Origin (FUO)

In some cases, a person will have a fever without an obvious cause.

A fever of unknown origin (FUO) is said to occur when:

• The temperature is equal to or higher than 101 degrees F on at least two occasions.
• The fever lasts for more than three weeks.
• The fever has no obvious source even after medical evaluation.
• The patient is not immunocompromised.

Sometimes the cause of an FOU may emerge after time, but often no cause is found.

When Fevers Are Dangerous

Except in rare circumstances, body temperature usually doesn't get very high or cause harm. Still, there are times when a fever can be a sign of a serious condition, and very high fevers can be dangerous. In these cases, it is important to get a medical evaluation.

If you have a persistent fever or recurring fevers, make an appointment to see your doctor.

Some children experience febrile seizures when they get fevers, particularly if they're above 101 degrees F. While these events are usually not dangerous, and they do not cause a seizure disorder, you should call your child's pediatrician for guidance about treatment.

A Word From Verywell

Fevers can be scary, but keep in mind that most of them won't hurt you or your child. They are a normal part of your body's response to illness. If you're worried, check with your healthcare professional and see what you can do to lower a high temperature or ease any discomfort it causes.