Not every virus gets its own “season,” but influenza—commonly known as the flu—is known to make its rounds each winter (and often in the fall and spring, too).

The flu is a contagious viral infection that attacks the respiratory system (throat, nose, and lungs). The degree of illness can range from mild to severe and can include fever, body aches, cough, headache, and sore throat.

Typically, the virus resolves on its own, but in extreme cases, or especially for the very vulnerable, including infants, the elderly, and those with compromised immune systems, it can lead to hospitalization—and even death.

According to the Centers for Disease Control and Prevention (CDC), the best way to protect yourself or your family from getting the flu virus is with an annual flu vaccine.

“Prevention is key,” says Annette Cameron, MD, a Yale Medicine pediatrician. “Get your flu shot. Even if the vaccine isn’t always the perfect match that year, it might at least reduce the severity of the flu if you get it.”

Common symptoms of the flu may include the following:

• Fever of 100.4 F or higher
• Aching muscles, especially in your back, arms, and legs
• Chills and sweats
• Headache
• Cough
• Runny or stuffy nose
• Sore throat
• Fatigue and weakness

At first, it may be hard to distinguish between a common cold and the flu. But one difference is that colds tend to come on somewhat gradually, whereas the flu usually develops more suddenly.

“Basically, you are sicker with the flu compared to a standard upper-respiratory infection. It’s like a cold on steroids,” says Dr. Cameron. “Your fever is higher. Your symptoms last longer. You are miserable and achy.”

Flu symptoms are also similar to those caused by COVID-19. Testing is necessary to know whether you have flu or COVID-19 (or another respiratory illness).

In rare cases, patients with flu might not have any symptoms, so one should not self-diagnose their illness based on symptoms alone.

The flu can affect everyone. But the following people are at a higher risk of developing serious complications related to the flu:

• People 65 and older
• People of any age with chronic conditions (asthma, diabetes, heart disease)
• Pregnant women
• Children younger than age 5 and especially younger than age

For the young and healthy, the flu may be unpleasant, but it isn’t typically life-threatening. Most people recover within a week or two. However, there can be serious complications for children and adults:

• Pneumonia or bronchitis. The flu virus can cause damage to your body’s defense system (including the membranes that line your airways and the cilia, which are tiny hairs that help keep the respiratory tract free of mucus). This damage makes patients more susceptible to other bacteria.
• Asthma flare-ups. Inflammation of the respiratory tract can cause irritation, which can lead to bronchospasms (a sudden constriction in the airways) and cause symptoms of asthma to worsen.
• An increase in the short–term risk of heart attack. It’s unclear why the flu sometimes leads to heart attack, but doctors suspect it has to do with the physiologic stress on the body from fighting the flu.

When someone infected with the flu virus sneezes, coughs, or talks, they release droplets containing the virus into the air. You can unknowingly inhale those droplets or come into contact with them by touching an object such as a doorknob, counter, or telephone and then touching your nose, eyes, or mouth. Once the virus enters your body, it moves into the respiratory tract and latches onto and enters your cells. Once a virus gets inside a cell, it uses the cell’s own machinery to make many copies of itself. These copies make their way out of the cell and then infect other cells. In this process, many cells die. Over the course of several days, the immune system fights off the virus, eliminating it from your body. (After your body successfully fights the virus, those cells will grow back.)

People infected by the virus are contagious starting a day before symptoms begin and until five to seven days after the onset of symptoms. People are most contagious during the first three to four days after symptoms first appear. Because it’s hard to know for sure if and for how long you’re contagious, it’s difficult to prevent the flu’s transmission to others, especially among families.

What’s even more problematic is that new strains of the virus appear often. This means that even if you’ve had the flu that season or received the flu vaccine, you can still get the flu if the antibodies your body has developed are for a different strain of the virus.

Often, your doctor will diagnose the flu based on your symptoms. There are also a number of tests that can detect the virus by examining specimens taken by swabbing the inside of your nose.

The most commonly used are the rapid influenza diagnostic tests (RIDTs). They can provide results within 10 to 15 minutes, but they can miss certain cases of flu. They do, however, appear to be more accurate at diagnosing flu in children compared to adults.

Typically, plenty of rest and drinking a lot of fluids—because a fever causes the body to lose more fluid than usual and can lead to dehydration—is all you need to do if you get the flu. Often, over-the-counter pain medications such as acetaminophen (Tylenol) or ibuprofen (Motrin) are all adults or children need to reduce fever or body aches. Parents are sometimes afraid to give their kids too much medicine—a concern Dr. Cameron understands.

“However, they often base the dose on their child’s age instead of their weight, and often kids weigh more than the typical dosing for their age. I tell parents to base it on weight because otherwise, they won’t get enough and they won’t feel better,” she says. “But if they’re unsure, they should call their physician, and ask them to calculate the actual dose.”

Sometimes, your doctor may prescribe an antiviral medication like Tamiflu (oseltamivir). If taken within 48 hours of symptom onset, the medication may shorten the course of illness by about a day and help prevent serious complications.

“We do give Tamiflu when we catch the flu early, but I tend to reserve it for kids who have other chronic illnesses, especially asthma, and for younger babies,” Dr. Cameron says. “For older kids who don’t have a chronic illness, or if they have had symptoms for longer than two days, I don’t bother because it comes with its own side effects and can cause upset stomach and vomiting.”

Doctors may prescribe other antivirals to treat the flu, including Relenza (zanamivir), Rapivab (peramivir), and Xofluza (baloxavir). These medications work by interfering with the virus’s ability to infect cells in the body.

Antiviral medications work best when they are taken within two days of the onset of symptoms, but they may be given outside of that timeframe for those who are hospitalized, have severe flu, or have an increased risk for complications.

Antivirals may also be an option for people who do not have severe illness and those who are low risk for complications provided the medications can be started within two days of symptom onset.

The CDC recommends annual flu vaccinations for everyone over the age of 6 months, including pregnant women.

Each year, the vaccine contains protection from the three or four influenza viruses anticipated to be the most common during that year's flu season. Researchers do their best to guess which virus strains will be circulating in the community by looking at other parts of the world (such as Australia) where the flu season has already occurred and matching the vaccine to combat those types.

However, because vaccine production takes time, the vaccines have to be produced well in advance of the actual flu season. This makes it difficult to match the virus with the vaccine perfectly every time, says Matthew Grant, MD, a specialist in infectious disease at Yale Medicine.

“Still, there really is no downside to getting the flu vaccine,” points out Erin Nozetz, MD, a Yale Medicine pediatrician. “You may have a sore arm and experience some body aches or low-grade fever, but that is a good thing because it means your immune system is responding to the vaccine. And we wouldn’t expect those symptoms to last more than a few days.”

Carl Baum, MD, a Yale Medicine pediatric emergency physician, agrees, and adds that some people falsely believe getting the vaccine actually causes the flu.

“It’s true that some people may have a mild reaction, like a fever, after getting the vaccine. But it usually happens within the first 24 hours of vaccination and then goes away,” he says. “What it means is that your body is reacting and doing what it is supposed to. People mistake the body’s natural response to the vaccine with actually getting the flu.”

That’s why physicians advise adults and children not to get the flu vaccine if they already have an upper-respiratory infection or otherwise don’t feel well. “If you get the flu vaccine while you’re sick, you might have an increase in your symptoms, and then everything gets blamed on the vaccine. The truth is you already had the infection, but the vaccine might temporarily make it a little worse,” he says.

Measures to decrease the possibility of spreading the flu include the following:

• Wash hands. Use soap and water and scrub for at least 20 seconds. Alcohol-based hand sanitizers are another option when washing your hands is not possible.
• Cover your mouth. To prevent the spread of droplets, cough, or sneeze into the crook of your arm or a tissue.
• Be mindful of crowds. Flu spreads quickly in busy places, including schools, offices, child-care centers, and on public transportation. If you’re sick, stay home for at least 24 hours after your fever abates in order to limit infecting others.

It’s hard to provide a definitive time frame, says Albert Shaw, MD, PhD, an infectious diseases doctor at Yale Medicine. “The CDC recommends getting it by the end of October and the thinking is that after you get vaccinated it can take a few weeks for your body to generate the antibodies,” he says.

The other thing to consider is how common the flu is in your community at the time, even though this can be difficult to predict. But there is one thing Dr. Shaw is sure of: It’s never too late for the vaccine to be effective. “The flu continues well into the spring,” he says.

Yale Medicine doctors are internationally recognized for their experience in treating viral and bacterial infectious diseases. We work with physicians from multiple departments to offer comprehensive care for patients. In addition, our clinicians are investigating the biological reasons why the flu vaccine might be more effective for some ages compared to others.