If You Notice This Symptom, It Could Be This Year’s Flu, Reveals an Expert Doctor

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The CDC expects a moderate flu season ahead, though it's still too early to know for sure.

There isn't one hallmark symptom that can reliably distinguish flu from COVID, RSV, or other respiratory infections.

But experts say there is one useful clue: Flu symptoms tend to come on hard and fast.

The U.S. Centers for Disease Control and Prevention (CDC) expects a "moderate" flu season ahead, though it's still too early to know exactly how this year's respiratory virus season will unfold. With multiple bugs expected to be going around at once, it can be hard to tell what's causing that sore throat, fever, or fatigue.

Unfortunately, there isn't one magic sign that reliably points to the flu … or any other seasonal infection for that matter. "Symptoms alone cannot distinguish different respiratory viruses," explains Majd Alsoubani, MD, MPH, an infectious disease physician at Tufts Medical Center. "Flu and COVID can cause fever, cough, body aches, and fatigue, and these symptoms also overlap with other respiratory infections that can circulate in the season."

While flu activity is still low in the U.S, there's typically a peak in January and February, says Suraj Saggar, DO, chief of infectious disease at Holy Name Medical Center in New Jersey. "Although that can depend on several factors, including weather—a milder fall may mean more people stay outdoors, giving the flu less time to circulate," he says. And, he adds, it is definitely not too late to get your flu shot if you haven't yet.

So how do you know if you have the flu? Ahead, infectious disease experts explain what you should be on the lookout for, what to do if you catch the flu, and how to best protect yourself from getting it in the first place.

The most distinctive characteristic of influenza this year? Experts say that rather than one standout sign, a more reliable clue is how quickly symptoms arrive.

"Flu symptoms usually come on suddenly, and people who have flu often first feel fever or chills, followed by cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and fatigue," Dr. Saggar says. "You may feel rundown or exhausted even before the fever hits." Unlike a cold, which develops gradually with a scratchy throat and mild congestion, flu symptoms can come on quickly, be more intense, and last longer, he adds.

Research shows that initial pile-on can be substantial, too. A July 2026 study published in BMC Infectious Diseases tracked 724 adults with the flu and found that symptoms hit especially hard in the first few days. At the start of illness, participants reported an average of about nine symptoms, with six that they rated from moderate to severe. The most commonly reported symptoms included runny or stuffy nose, cough, fatigue, headache, body aches, fever, chills, and sore throat.

Fever is common with the flu—flu strains can cause fevers between 102°F and 104°F, accompanied by severe chills that leave you alternating between freezing cold and burning up. But the CDC says it's important to note that not everyone with influenza develops a fever.

If you are burning up, however, Dr. Saggar says it's crucial to stay hydrated and monitor your temperature. Fevers can typically last three to four days, according to the CDC, but if it persists—or it breaks and then returns—contact your doctor. A fever that comes back or lasts more than five to seven days could signal a secondary bacterial infection or flu complications, which is especially concerning for high-risk populations, says Dr. Saggar.

"If you think you have the flu, it's a good idea to go get tested because it helps us gather accurate data and also because we have antiviral treatments that can lessen the severity and duration by one to three days," Dr. Saggar says. And while that might not sound like much in the grand scheme of things, if you've ever had the flu, then you know that one less day of feeling miserable is well worth the trip to the pharmacy.

Because antivirals work best when started within one to two days after symptoms begin, reaching out early matters. In fact, the CDC recommends starting antiviral treatment right away for people who are hospitalized, seriously ill, or at higher risk of complications—and that's even if the flu is only suspected, not yet confirmed. People at higher risk of developing serious flu-related complications include those 65 years and older, pregnant women, children younger than 5 years, and people of any age with certain chronic medical conditions such as asthma, diabetes, or heart disease.

If you develop flu symptoms, there are steps you can while monitoring your symptoms at home. Stay hydrated with water, herbal tea, or electrolyte drinks, as fever increases fluid loss through sweating. Use over-the-counter fever reducers such as acetaminophen or ibuprofen—if recommended by your doctor—and apply a cooling compress to help lower body temperature.

"Rest is essential—your body needs energy to fight the infection," says Dr. Saggar. And don't be surprised if you still don't feel quite back to normal once the worst passes. In the 2026 BMC Infectious Diseases study, severe symptoms dropped off over the first several days, but some participants still reported lingering milder symptoms, such as fatigue, for up to four weeks.

Also, make sure to stay home for at least 24 hours after your symptoms have improved and you've been fever-free without medication. People are contagious for about a day before symptoms start and can continue spreading infection for up to a week after getting sick. That's why the CDC advises taking added precautions, such as masking, practicing good hand hygiene, and physical distancing for five days after you're feeling better, especially around people vulnerable to severe illness.

While most people recover from the flu within a week or two, certain symptoms warrant immediate medical attention. Dr. Saggar recommends seeking emergency medical care if you experience any of the following:

Difficulty breathing or shortness of breath

Persistent pain or pressure in the chest or abdomen

Fever or cough that improves but then returns or worsens

While experts predict this flu season will be more moderate, there are a few key things to keep in mind to maximize your odds of staying healthy. As Dr. Saggar says, "The best defense remains prevention and having healthy habits overall. Get your annual flu shot if you haven't already, wash your hands frequently, and stay home when you're sick."

Suraj Saggar, DO, is the chief of infectious disease at Holy Name Medical Center in New Jersey and is a nationally-recognized expert in the diagnosis and treatment of infectious diseases, including influenza. He is an adjunct clinical associate professor at Touro College of Osteopathic Medicine in New York City. He is board-certified in infectious disease and is a Fellow of the Infectious Disease Society of America.

Majd Alsoubani, MD, MPH, is an infectious disease physician at Tufts Medical Center and assistant professor of medicine at Tufts University School of Medicine, with expertise in infections in immunocompromised hosts and antimicrobial stewardship.

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