What is the flu? 17 things to know about flu symptoms, flu shot side effects and more

According to the Centers for Disease Control and Prevention, the United States experiences flu epidemics every year.

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Since 2010, the CDC estimates the flu has resulted in between 9.2 million and 35.6 million illnesses.

Here's what you should know about the flu, the flu shot and more from experts in the field.

What exactly is flu?

According to the CDC, the flu is "a contagious respiratory illness caused by influenza viruses."

Influenza viruses directly infect the nose, throat and lungs. It can cause mild to severe illness and can even be fatal.

How does the influenza virus start?

Influenza actually originates among birds and other animals in Southeast Asia and then spreads to Europe and, lastly, the U.S., William Schaffner, chair of Vanderbilt Medical School's department of preventative medicine, told ABC News.

How does the virus spread among humans?

Humans spread the virus we have in our throats to others when we touch each other, especially noses and mouths, Schaffner said.

Why are we so susceptible to the virus?

It's because the virus comes in different forms, and several forms, every year, according to Schaffner.

Flu viruses are most common during the fall and winter, though seasonal flu viruses can be found year-round in the U.S.

According to the CDC, virus activity often increases in October and peaks between December and February. It can last through May.

What are the symptoms of flu?

Someone who is exposed to the flu virus may begin to experience the following symptoms about one to four days after exposure.

From the CDC:

  • Fever or feeling feverish/chills -- Note that not everyone will experience a fever.
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Muscle or body aches
  • Headaches
  • Extreme fatigue
  • Some may experience vomiting and diarrhea, though this is more common in children than adults.

How long is someone contagious after contracting the flu?

An infected person may be able to infect other people and spread the illness one day before they even begin to experience symptoms.

According to the CDC, the period of contagiousness can continue between 5-7 days after becoming ill.

The period of contagiousness could last even longer for those with weakened immune systems, especially young children.

Who is most at risk of getting the flu?

According to the World Health Organization, pregnant women, older people, young children and people with certain chronic health conditions -- such as asthma, diabetes and heart disease -- are most at risk.

What does the flu vaccine do?

The seasonal vaccine protects against the form of the virus scientists are predicting will be most comon during the upcoming flu season.

According to CDC experts, the traditional three-component flu vaccine protects against three flu viruses: H1N1, H3N2 -- both influenza A viruses -- and an influenza B virus.

The quadrivalent flu vaccine protects against those three plus an additional influenza B virus.

Who should get a flu shot?

CDC experts recommend everyone six months old or older receive a flu shot every year.

What about the nasal spray flu vaccine?

The CDC does not recommend the nasal spray flu vaccine during the 2017-2018 flu season. Injectable influenza vaccines are recommended instead.

Can people with egg allergies receive the flu shot?

Yes. CDC experts recommend those with severe egg allergies be vaccinated in a medical setting with a health care provider present.

Should pregnant women receive the flu vaccine?

Yes. Experts recommend pregnant women get the shot during any trimester of their pregnancy to protect both themselves and their babies.

Will you get the flu from the flu shot?

No, the vaccine cannot cause flu, according to the CDC.

What are the side effects of a flu shot?

Common mild side effects may include soreness or swelling, headaches, nausea, muscle aches and fever.

The injection, like other injections, can occasionally cause fainting.

When is the best time to get the flu shot?

It takes about two weeks after getting the shot for the vaccine to protect against influenza.

That's why the CDC recommends people get vaccinated by the end of October, before the flu season starts to get bad.

If October has passed, however, it is still beneficial to get vaccinated, even into January or later.

How much does a flu shot cost and where can I get one?

Under the Affordable Care Act, most people with insurance who go to a health care provider covered by their plan should receive the flu vaccine for free.

Pharmacy chains, universities and other corporations have started offering the vaccines either for free or at low costs.

Use the CDC's Flu Vaccine Finder below to find a clinic near you. Just enter your ZIP code and hit "Go."