It’s never fun to have a fever, a symptom of infection that causes chills, sweats, body aches, weakness and loss of appetite, but here is the good news: A fever usually means that your body is doing its job to make you well again.
“Fevers make it harder for bacteria and viruses to survive,” says UNC Health family medicine physician Sarah Ruff, MD. “It’s a sign that your body is fighting off the infection.”
Everyone’s normal body temperature may be a little bit different, but the definition of fever is the same. For adults, an oral temperature of 100.4 F (38 C) or higher is a fever. (If you have children, talk to your pediatrician about fevers in babies or young children, as the measurement and concerns for this age group are slightly different.)
Here’s what you need to know about fevers, including how to feel better.
Causes of a Fever
The hypothalamus, a gland in the brain, is the body’s thermostat. When the body detects infection, it turns the thermostat up.
“Fever-inducing substances in the body called pyrogens are released, triggering the hypothalamus in the brain to raise your body temperature,” Dr. Ruff says. “Blood vessels in the skin constrict or get smaller to decrease heat loss, and muscles contract, causing shivering, which produces heat.”
That heat is a sign to your immune system to get to work, if it hasn’t already, while also creating an inhospitable environment for the infection. Externally, you may have symptoms like sweating, chills, shivering, chattering teeth or body aches.
Your immune system can fight infection without reaching a fever point. Your temperature may be only slightly elevated, so that you don’t experience or notice as many signs of it.
Fever is a symptom of many illnesses, including viral infections such as flu and COVID-19, and bacterial infections such as food poisoning, ear and sinus infections, and urinary tract infections.
“Some vaccines can cause a brief fever because when they introduce germ components into the body, it makes the body think it needs to increase the immune response to fight the disease,” Dr. Ruff says.
Most of the time, fever is just one of the symptoms we’re experiencing, making it easy to know the cause. Talk to your doctor if you’re having unexplained fevers.
“Fever could be a sign of cancer, with your body trying to fight off something foreign that’s not supposed to be there,” Dr. Ruff says. “If you’re having a fever by itself, without any other obvious cause or symptoms, or a fever with symptoms like unexplained weight loss, fatigue, malaise, night sweats or any other symptom of cancer, contact your doctor.”
How to Treat a Fever and Feel Better
Because a fever helps to trigger your immune system, you may be wondering if you should treat this symptom or endure it in the hopes of fighting an infection faster.
“You’re not going to alter the time that you’re sick by treating or not treating a fever,” Dr. Ruff says. “Your body has to go through the infection, so you can’t sweat it out or slow the course of getting better by taking medicine. It will be the same amount of time.”
A fever usually resolves on its own, so your decision to treat this symptom may depend on how bad you feel.
Acetaminophen, or Tylenol, is a fever reducer. Note that many over-the-counter cold and flu medications include acetaminophen as one of the ingredients, so you may be able to lower your fever as you address congestion or your cough. Dr. Ruff says acetaminophen is more effective than ibuprofen at lowering a fever but that ibuprofen can help with the body aches and pains you may experience during a fever.
“You can also put a cool washcloth on your head and neck or take a lukewarm—not hot, not cold—bath or shower to help bring a fever down,” Dr. Ruff says. “It’s also important to rest and sleep and give the body time to fight off what it needs to fight off. This isn’t a time for exercise.”
It’s also not a time for going out—if you have a viral infection that can be spread from person-to-person, a fever is a sign that you’re contagious.
“A urinary tract infection isn’t contagious, but for viral things like flu, RSV, pneumonia, COVID-19, you are most contagious when you have a fever,” Dr. Ruff says. “That’s why the recommendation is that you wait 24 to 48 hours after your last fever without the use of medication to bring it down before you return to school or work.”
You may have heard the expression “feed a cold, starve a fever,” suggesting that you don’t have to eat while you have a fever. Dr. Ruff says that while a fever can cause a loss of appetite, there’s no truth to this saying.
“When you have a fever, the body is burning a lot of calories and energy to get the temperature that high,” she says. “But even if you don’t feel like eating, you have to drink something. If you’re not able to take in fluids while you’re sick, that’s a worrisome sign, because you can get dehydrated. Always focus on hydration, and it will help you feel better.”
When to Go to the Doctor for a Fever
For most people, a fever in combination with other recognizable symptoms should not cause additional worry. It doesn’t indicate more severe disease, and while you may need more rest than usual, it won’t cause adverse effects. You may want your doctor’s guidance on treating the infection that is causing the fever, so Dr. Ruff recommends using an online patient portal or calling your doctor’s office for specific advice.
“If you’ve had a fever for longer than five days, then you should go to the doctor,” Dr. Ruff says. “You may have a bacterial infection that requires antibiotics.”
If you have fever and severe or emergency symptoms, such as difficulty breathing, chest pain, pain with urination or gastrointestinal discomfort, or if you have not been able to drink anything and may be dehydrated, then you should seek medical care. Depending on the symptom, you may be able to contact your doctor’s office or visit urgent care; for emergency symptoms, such as an inability to breathe, go to the emergency department.
A fever of 104 F and above is considered high.
“If you have a high fever, first take acetaminophen,” Dr. Ruff says. “If a high fever doesn’t go down in about 30 minutes after medicine, reach out to your doctor or go to urgent care. Most fevers don’t get that high with typical infection, so we’d want to evaluate why it’s so high and why it’s not coming down. It could be a sign of something more serious going on.”
Dr. Ruff says many people can tell when they have a fever because they know how the symptoms feel in their body, but you may not be able to tell how high your fever is, so be sure to measure your temperature with an oral thermometer.
Some People with Fevers Are at Higher Risk of More Serious Illness
For most adults, a fever lasting fewer than five days with no other severe symptoms and with a clear cause is not a reason for concern. There are some exceptions.
“If you are pregnant or postpartum, tell your doctor if you have a fever,” Dr. Ruff says. “When you’re pregnant, a fever could increase the temperature in the uterus and, in rare cases, affect the fetus, and we’d just want to make sure nothing else is going on. If you’re postpartum, it could be an infection in your uterus or breast that needs to be treated, or it could be a sign of a blood clot, and there’s a high risk of blood clots postpartum.”
If you have cancer and are undergoing chemotherapy, your doctor will want to know if you have a fever as it could indicate that your blood cell count is low.
People with autoimmune conditions or who take medications that affect their immune systems also need to contact their doctors if they have a fever; when you’re immunocompromised, there is a concern that you could develop a more severe infection, such as sepsis.
If you’re age 65 or older, you’re considered immunocompromised because of your age. Dr. Ruff notes that fever can present differently in older adults.
“An older patient can present with delirium or altered mental status when they have a fever,” she says. “They may start to act confused, or not like themselves. That might be the only symptom, so be sure they see a doctor and have a work-up for infection.”
Worried about a fever or any other symptom? Talk to your doctor. If you need a doctor, find one near you.