There are two main types of human flu viruses. Type A and Type B are the flu viruses that are responsible for the illnesses we see during flu season.

The flu can make you feel quite ill for a few days or up to 2 weeks. People can die from complications caused by the flu.

In a typical year, about 8 out of 100 people in the United States get sick with the flu. Flu season can begin as early as October and peak anywhere from late December to early April.

The best way to protect yourself from the flu is to get vaccinated each year as soon as the flu vaccine is available in your community.

Flu vaccines are available from 11 am to 2 pm Tuesdays through Saturdays at Northwestern Medicine Immediate Care Centers for patients age 3 years and older. Flu vaccines are also available by appointment at your primary care physician’s office. You can also get the flu vaccine at locations in your local community. Vaccines.gov allows you to search for a vaccination clinic by ZIP code.

• The flu vaccine, or flu shot, is given through a needle in your arm.
• The vaccine contains killed virus. That means it cannot give you the flu.
• It can be used for all people 6 months or older.

One reason the flu remains a problem is because the viruses change, or “mutate,” regularly. This helps the virus “hide” from the immune system of both children and adults.

The process works like this:

1. A person gets infected with an influenza virus.
2. Their immune system “sees” the virus and learns how to fight against it. It does this by developing “antibodies.” Antibodies destroy the virus in the body.
3. As the virus spreads in the environment, it changes.
4. During the next flu season, the same person gets infected again.
5. The "old" antibodies no longer recognize the "new" virus. The person gets sick, and their body makes new antibodies to fight the new virus.

Older antibodies can give some protection against getting the flu again. Companies make new vaccines each year to help protect against the influenza virus strains expected to cause the most flu cases that year.