This Is the Most Common Cause of Fever, According to Doctors


Having a fever can be uncomfortable, upsetting, and even a little scary, especially these days. But, while the odds are pretty good that you've had a high temperature at some point in your life before, you may be a little fuzzy on what, exactly, causes fever in the first place.

Having a fever that won't quit should at least prompt a call to your doctor, but it's understandable that you might want to have at least some idea of what's happening in your body. Here's what causes fever, and what you can do about it.

What is a fever, again?

At its core, a fever is a body temperature that's higher than normal. While you can have an elevated temperature if you find that you're running a little hotter than usual, you don't technically have a fever unless you have a temp of at least 100.4 degrees Fahrenheit, according to the Centers for Disease Control and Prevention (CDC). Also, "it's been debated by the medical community, but it's generally thought that a normal body temperature is at or around 98.6 degrees," Kathryn Boling, MD, a primary care physician at Baltimore's Mercy Medical Center, tells Health.

RELATED: What Temperature is Considered a Fever in Adults?

What happens in your body when you have a fever?

A fever is usually a sign that your body is trying to fight an illness or infection, according to the US National Library of Medicine's MedlinePlus resource.

What actually happens in your body when you have a fever can get a little complicated but, in general, "it's a sign that the immune system is fighting something, and that all of the body's systems are working to prioritize the health of vital organs," Roshi Gulati, MD, a family medicine physician at Northwestern Medicine Huntley Hospital, tells Health.

When you have a fever, your body's hypothalamus, a small region of your brain that act as an internal thermostat, among many other things, signals for your body temperature to increase, Laura Miller, MD, MPH, a family medicine physician with the University of Minnesota Medical School and University of Minnesota Physicians, tells Health.

At that point, "there is a cascade of events that are triggered, including clamping down on blood vessels in the peripheral parts of the body to shunt blood to more important organs and decreasing heat loss from the skin," Dr. Miller says.

"An increase in body temperature can slow bacterial growth, improve the defense capabilities of your white blood cells, and stimulate generation of biochemical reinforcements," Arindam Sarkar, MD, a primary care physician and assistant professor of family and community medicine at Baylor University, tells Health. Basically, a fever is usually a sign that your body is trying to fight off an invader.

Most of those bacteria and viruses can survive OK in your body when your temperature is normal, Erik Blutinger, MD, an emergency medicine physician at Mount Sinai Queens, tells Health. "But if you have a fever, it may be harder for them to survive," he says.

RELATED: 8 Reasons You Have the Chills—Even if You Don't Have a Fever

What usually causes a fever?

There is a massive list of potential health issues that can cause a fever, but they can generally be lumped into the following categories, per Medline Plus:

  • An infection
  • Certain medications
  • Heat illness
  • Cancers
  • Autoimmune diseases
  • Some childhood vaccines

Still, an infection, whether it's bacterial, fungal, or viral, is usually the most common cause of fever, Dr. Gulati says. More specifically, "cold viruses are the most common cause of fever, and before COVID-19 we'd tell people to just let it run its course," she says. Those cold viruses are typically coronaviruses, she says, but "COVID-19 is far more dangerous than the others we see."

Dr. Sarkar agrees: "Overwhelmingly, the majority of fevers are caused by viral and bacterial infections," he says. "Rarely, systemic rheumatic diseases and cancers can cause prolonged, unexplained fever."

RELATED: What Is a Low Grade Fever? We Asked Experts to Explain

What should you do if you develop a fever?

First, it's important to know this: "If you feel relatively OK, you don't necessarily have to take anything for it," David Cutler, MD, a family medicine physician at Providence Saint John's Health Center in Santa Monica, Calif., tells Health. But, if you're uncomfortable or your fever is particularly high, you might want to take a fever-reducing medication like acetaminophen or ibuprofen, Dr. Cutler says. Staying hydrated is also important, he adds.

You'll want to take note of your other symptoms, too, Dr. Boling says—that can help clue your doctor in to what might be causing your fever. And, given that fever can be a sign of COVID-19, it's a good idea to call your doctor about next steps if you happen to develop one, Dr. Gulati says. "COVID-19 has changed everything when it comes to caring for patients who have fevers," she says. "We usually test for COVID-19 at the same time we're considering other causes of their illness."

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