The flu is a contagious respiratory illness that can cause symptoms including fever, coughing, chills, body aches, and fatigue. Flu season strikes every year, and the virus can spread rapidly in schools and workplaces.

Some people who get the flu recover without complications in about one to two weeks. But the flu can be dangerous for young children and people 65 years old and up. Some flu-related complications are also life-threatening.

It’s important to arm yourself with as much knowledge as possible. This way, you know how to better protect yourself.

While many people get the flu at least once in their lifetime, you might not know everything about this illness. Here are 10 facts about the flu you should know.

When you think about the flu virus, you may assume that it only strikes in the winter. While it’s true that flu season can peak in the winter, you can get the flu in the fall and spring, too.

Some people get seasonal flu as early as October, with infections continuing through May.

The flu is highly contagious partly because it’s possible to pass the virus on before you become sick. According to the Centers for Disease Control and Prevention (CDC), you can infect someone with the virus one day before your symptoms start.

You’re most contagious within the first three to four days of becoming ill, although you may remain contagious for up to five to seven days after you become sick.

It’s important to avoid close contact with others to prevent passing the illness to another person.

The onset of flu symptoms can happen rapidly. You may feel fine one day, and be unable to do anything one or two days later due to your symptoms.

Sometimes, the onset of symptoms occurs as early as one day after exposure. In other cases, some people don’t show symptoms until four days after exposure to the virus.

Getting a seasonal flu vaccine is one of the best ways to protect yourself against the influenza virus.

But it’s important that you get your shot early in the season. The flu shot is effective because it helps your body develop antibodies to protect itself against the virus. It takes about two weeks for these antibodies to develop, though.

If you’re exposed to the virus within two weeks of getting a vaccine, you may still get sick. The CDC recommends getting a flu vaccine by the end of October.

The predominant flu viruses circulating this season will differ from next year’s viruses. This is because the virus undergoes changes each year. Therefore, you’ll need a new vaccine every year to protect yourself.

One misconception is that the flu vaccine causes the flu. One variety of the flu shot does include a severely weakened form of the flu virus. It doesn’t cause real infection, but it allows your body to develop necessary antibodies. Another variety of the flu shot only includes dead, or inactivated, virus.

Some people do experience mild flu-like symptoms after getting a vaccine. This can include a low-grade fever and body aches. But this isn’t the flu and these symptoms typically only last one to two days.

You may also experience other mild reactions after getting the flu vaccine. This includes brief soreness, redness, or swelling at the injection site.

The flu vaccine is especially important if you’re at risk for flu-related complications. Complications are more likely to occur in certain groups, such as:

  • people who are at least 65 years old
  • young children, especially those under 2 years old
  • pregnant women and women who are up to two weeks postpartum
  • people who have a weakened immune system
  • people who have chronic conditions
  • Native Americans (American Indians and Alaska Natives)
  • people with extreme obesity, or a body mass index (BMI) of at least 40

However, anyone can develop severe complications.

The flu virus can also trigger secondary infections. Some infections are minor, such as an ear infection or a sinus infection.

Serious complications can include bacteria pneumonia and sepsis. The flu virus can also worsen chronic conditions such as congestive heart failure, asthma, and diabetes, and can lead to heart attacks and stroke.

Be mindful that it’s possible to get the flu after receiving a vaccination. This can happen if you become infected with the virus before your vaccine is effective, or if the flu vaccine does not provide adequate coverage against the predominant circulating virus.

Additionally, you can become sick if you come in contact with a strain of the virus that’s different from the one you were vaccinated against. On average, the flu vaccine reduces the risk of illness by between 40 to 60 percent.

The CDC currently recommends either an injectable flu vaccine or a live attenuated intranasal flu vaccine.

The flu vaccine isn’t one-size-fits-all. There are different types of vaccines available.

One type is the trivalent flu vaccine. It protects against three flu viruses: influenza A (H1N1) virus, influenza A (H3N2) virus, and an influenza B virus.

Another type of vaccine is known as quadrivalent. It protects against four flu viruses (both influenza A viruses and both influenza B viruses). Some versions of the quadrivalent flu vaccine are approved for all age groups, including children at least 6 months old and pregnant women.

Other versions are only approved for adults between the ages of 18 and 64, or adults 65 and up. Your doctor can help determine which one is right for you based on your age and health.

There’s a belief that you can’t get a flu vaccine if you’re allergic to eggs. It’s true that some vaccines contain an egg-based protein, but you may still be able to receive the flu vaccine. You’ll just need to talk with your doctor before getting a shot.

Your doctor may administer a vaccine that doesn’t contain eggs, or have a doctor that specializes in allergies administer the vaccine so they can treat any potential reaction.

The flu can range from mild to severe, so it’s important that you recognize symptoms early and start treatment to avoid complications. The more you understand about the virus, the easier it’ll be to protect yourself and your family.