Causes of Fevers

A fever is a natural part of your body's defense against illness, rather than an illness itself. While infections such as the flu are the most common causes of fever, your temperature may increase due to medication use, inflammation, vaccinations, and other factors. In some cases, 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. But learning more about possible fever causes can help you get a better sense of the wide range of reasons the body reacts in this way—and why, in some cases, medical attention is (or is not) necessary.

How Fevers Happen

A higher temperature is believed to kill off temperature-sensitive pathogens, like viruses (i.e., rhinovirus) and bacteria (such as Streptococcus) that make you sick. Most pathogens live well and multiply at the normal body temperature of 98.6 degrees F, but cannot survive higher temperatures. In essence, in triggering a fever, the immune system uses heat as a weapon against illness.

But numerous other substances and processes can lead to fever even when the immune system isn't attacking a pathogen.

The specific physiological process behind a fever depends on what's triggering it.

Infections

An infection of a virus, bacterium, or fungus can make you sick and lead to a fever. These illnesses can include the ones that likely occur to you first when you think a fever, such as the flu or strep throat, but they can also be things like Lyme disease, kidney infections, ear infections, and appendicitis.

When your immune system recognizes something as an invader, it releases chemicals called pyrogens into your blood. Pyrogens travel to a region in your brain called the organum vasculosum lamina terminalis, where they produce chemical messengers called prostaglandins.

These then 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 cranked up to essentially "cook" the pathogen to death.

Some pathogens contain pyrogens as well, which is why certain illnesses are more associated with fever than others. Escherichia coli (E. coli), pseudomonas, and enterobacter are examples of pyrogen-containing pathogens.

Immunizations

Vaccines can sometimes trigger a mild fever. That's because they purposefully introduce small amounts of a pathogen (but not enough to give you the illness) so your immune system can learn to recognize and fight it.

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 is an important part of the healing process and is regulated by your immune system. 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:

Autoinflammatory diseases linked to fevers include:

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, several possibilities exist. First, cancer causes inflammation. Additionally, doctors believe some cancer cells themselves may produce pyrogenic substances.

Other diseases that are both inflammatory and may involve pyrogens include:

Sometimes, blood clots are associated with fever. Doctors don't yet know for certain how blood clots lead to fevers, but one possibility being explored is that a clot leads to inflammation. Another possibility involves an immune reaction caused by problems with blood vessels.

Surgery can also lead to inflammation and inflammation-related fevers.

Medications, Illegal Drugs, and Alcohol

Certain drugs can act like pyrogens and thereby trigger a fever. An example is the class of antidepressants called selective serotonin reuptake inhibitors (SSRIs).

These medications are believed to change the action of chemical messengers (neurotransmitters) in your brain, which can trigger the hypothalamus to increase your temperature.

Heavy alcohol use affects neurotransmitters as well. 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.

Abuse of drugs in the amphetamine class—including the illegal drugs methamphetamine, Molly, and bath salts—can also increase body temperature through complex processes involving numerous hormones. These drug-induced fevers are believed to be involved in damage to the brain, liver, and muscles.

Brain Injury

Traumatic brain injury can cause fever if the hypothalamus is damaged.

Fevers of Unknown Origin

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 higher than 101 degrees F on several separate occasions.
  • The fever lasts for more than three weeks.
  • The fever has no obvious source even after medical evaluation of at least three doctor's visits or three days in the hospital.

If and when a cause is eventually found, FUOs typically fall into one of four classifications:

  • Classic: Infection, cancer, or a collagen vascular disease such as ankylosing spondylitis, rheumatoid arthritis, lupus, or scleroderma
  • Nosocomial: Clostridium difficile enterocolitis, drug-induced fever, pulmonary embolism, septic thrombophlebitis, sinusitis
  • Immune deficient: Opportunistic bacterial infections, aspergillosis, candidiasis (yeast infection), herpes virus infection
  • HIV-associated: Cytomegalovirus, mycobacterium avium-intracellulare complex, pneumocystis carinii pneumonia, drug-induced fever, Kaposi's sarcoma, lymphoma

The mechanism of action behind these FUOs depends on the cause, but most of them deal with infection or inflammation.

When Fevers Are Dangerous

Except in rare circumstances, one's body temperature will not go so high that it will cause harm. Still, there are times when a fever will require medical evaluation.

For example, while a fever of 103 degrees F in adults and children over age 4 is considered high-grade, it's considered potentially dangers at 104 degrees F and dangers above 106.7 degrees F (although fevers that high are rare).

The younger the child, the lower the threshold for getting medical attention. For example, a temperature higher than 102.2 degrees F in kids ages 3 months to 3 years should prompt a visit to the doctor, as it could indicate an infection or illness that needs to be treated. Younger children, however, do not often develop fevers unless they have a serious illness, so they should see a doctor for anything higher than 100.3 degrees F.

Some children experience febrile seizures when they get fevers, particularly if they're above 101 degrees F. Although these seizures are generally not dangerous and don't cause permanent damage, they're frightening and should be evaluated by a doctor right away.

Get emergency medical help if your child has any of these symptoms:

  • Inconsolable crying
  • Extreme fussiness or irritability
  • Sluggishness
  • Trouble waking up
  • A rash or new spots that look like bruises
  • Blue lips, tongue, or nails
  • Bulging or sunken soft spot
  • Stiff neck
  • Severe headache
  • Limpness, refusal to move
  • Trouble breathing even with a clear nose
  • Leaning forward and drooling
  • Seizure
  • Belly pain that's moderate to severe

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 doctor or pediatrician and see what you can do to lower a high temperature or ease any discomfort it causes.

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