Fever is a common symptom of illness. The Centers for Disease Control and Prevention (CDC) defines fever as a temperature above 100.4° F, though children often run higher fevers than adults.
Many types of infection can cause fever, as your body will increase temperature to effectively fight off the virus or bacteria causing the infection. These fevers will usually resolve themselves as your body works to get rid of the foreign invader.
Here's how you can figure out exactly what's causing your fever and whether it's necessary to seek medical attention.
Infections can cause fever
Infections are the most common cause of fever. While symptoms can vary, these commonly include respiratory infections like the flu, pneumonia, or bronchitis, as well as gastrointestinal infections like food poisoning or a stomach virus.
Fever occurs when pyrogens are released into the bloodstream. Pyrogens are naturally-occurring substances made in part from proteins, and can be produced by bacteria or a virus that is causing an infection, or by the body's immune system in response to infection.
The pyrogens affect the hypothalamus, the area of the brain that controls temperature. "Then, the body raises the metabolic rate in muscles and the liver to increase the temperature," says Alex Lickerman, MD, a primary care physician. The increased temperature can kill the germs causing illness.
In addition to common viruses like the flu, bacterial infections can also cause fever, as can inflammatory diseases like rheumatoid arthritis. Heatstroke can also cause fever. Sometimes, fevers are unexplained, Lickerman says.
How to figure out the cause of your fever
The severity of your fever and its associated symptoms may depend on what's causing it. Again, there are many different types of infections that can cause a fever, and many other reasons for feeling feverish, so it's best not to jump to conclusions when judging your symptoms.
But if you're worried that your fever may indicate COVID-19, here's how you can help figure it out, compared to other common conditions:
Influenza usually includes a higher fever (103 to 105° F) that lasts for 3 to 5 days, Lickerman says. The fever is often accompanied by chest discomfort, chills, and body aches.
COVID-19 symptoms usually include a fever similar to that caused by the flu. COVID-19 fevers generally start 3 to 5 days after other symptoms, like cough and shortness of breath, and last 5 to 7 days, Lickerman says.
With a common cold you might have no fever at all, or a low-grade fever, meaning under 100.4° F, says Lickerman. Other symptoms include sneezing, sore throat and coughing.
While fever isn't usually cause for concern, it indicates an underlying condition that you should monitor. "Fever itself is rarely dangerous unless it's very high — greater than 104° F — but it always indicates a medical problem," Lickerman says.
When to see a doctor for your fever
Most fevers, especially those caused by a virus, will resolve on their own within 1 to 7 days if left untreated, says Georgine Nanos, MD. "Nearly everyone will develop a fever at some point in their lives. The challenge is to know when to be concerned," Nanos says.
In general, infants 12 weeks or younger should see a doctor any time they have a fever greater than 100.4° F, even if they don't have other symptoms. Children should see a doctor if their fever lasts 3 to 5 days, is reoccurring, or is 104° F or higher.
Adults should see a doctor if their fever is higher than 104° F. Anyone experiencing a fever along with confusion, a rash, or trouble breathing should consider seeking medical attention.
If you believe your fever is caused by COVID-19, you should take measures to self-quarantine and stay away from family members and everyone else.
"It will be difficult for you to know if the symptoms you are experiencing are from COVID-19, and we are not recommending that you seek definitive testing, but rather base treatment on the severity of your symptoms," says Jordana Haber, MD.
The key is to determine whether your symptoms are mild, moderate, or severe. Mild to moderate cases of COVID-19 can be treated at home, following CDC guidelines. However, if you are experiencing trouble breathing or symptoms that you find particularly concerning, you should seek medical care, but first call ahead to your provider.
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