The Early Flu Symptoms You Should Know About, According to Infectious Disease Experts

This year more than ever, you've likely been monitoring anything that feels even a little off in your body—like taking your temperature if you feel slightly hotter than usual, or questioning whether a stray sneeze is allergy related or something more serious. Now, with flu season fast approaching, you may be on even higher alert, keeping an eye out for any potential warning signs or early symptoms of an influenza infection. (Tip: You can reduce some of that worry by getting your flu shot ASAP.)

But when it comes to the flu, there's something important to keep in mind regarding symptoms: There may not be any of those early symptoms that can help you reduce severity before it fully starts. "It comes on pretty rapidly, usually within the first day of the flu," Jennifer Lighter, MD, a pediatric infectious diseases doctor at NYU Langone, tells Health. In fact, quickness is kind of a hallmark of the flu—it can make you feel really awful, really fast. "The flu is usually described as, 'I got hit by a truck.' People get ill extremely quickly," adds Waleed Javaid, MD, director of infection prevention and control at Mount Sinai Downtown in New York.

That said, there are still some technically early flu symptoms—as in, those symptoms that appear seemingly overnight—that you need to be aware of, according to the Centers for Disease Control and Prevention:

• Fever or chills
• Headache
• Cough
• Sore throat
• Runny or stuffy nose
• Fatigue
• Muscle or body aches
• Diarrhea or vomiting, mainly in children

Those symptoms come on extremely fast—people usually notice them one to two days after being exposed to the influenza virus, Sherif Mossad, MD, an infectious disease specialist at Cleveland Clinic, tells Health. "Within 24 to 48 hours you could have those symptoms," Dr. Javaid adds.

The challenge this year, doctors warn, is telling the difference between flu and COVID-19. While the CDC says that flu symptoms can often come on quicker than those associated with coronavirus, there's still a possibility that someone could experience COVID-19 signs as early as two days after infection. And unfortunately, the chances of being able to tell which illness you’re dealing with are slim to none. “It’s going to be impossible,” Dr. Javaid says. “Even as a clinician I can’t say, looking at someone, ‘They have flu, but this person has COVID,’” Dr. Lighter says.

For this reason, experts are urging everyone to be sure to get their flu shot this year. The flu shot is effective not only in keeping you and those around you safe, but also in preventing logistical nightmares: Dr. Javaid warns that if someone develops, for example, a fever because they didn’t get the flu shot and have contracted the flu, this could cause their school or office to have to go into quarantine since fever is also indicative of COVID-19.

“The best bet all of us have right now is really good control of influenza,” Dr. Javaid says. “It otherwise creates a lot of other problems in a society. For example: I went to school, I was exposed to flu, I was never vaccinated, [I have] flu-like symptoms. The entire school gets shut down because I didn’t get my flu shot.” Getting the flu shot can also help prevent an overload in your local ER. “Even if the flu shot is marginally effective, this would help prevent overloading the healthcare system, particularly if an effective vaccine for COVID-19 does not become available during the coming flu season,” Dr. Mossad says.

So, what should you do if you develop symptoms that signal you could have the flu or COVID-19? Seek medical attention, Dr. Lighter says—but definitely give your doctor a call first to not only protect your health but the health of others you may come into contact with. The exception to this is if you're experiencing severe symptoms, Dr. Mossad says. But “depending on your age and background health, you may be advised accordingly" regarding treatment and next steps, he says.