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Doctor giving a man the flu shot

Flu (Influenza)

Northwestern Medicine is dedicated to preventing flu (influenza) epidemics and complications by providing vaccinations, resources and information.

What Is Influenza?

Influenza, or flu, is caused by one of several types of viruses that infect the nose, throat and lungs. Flu can make people feel quite ill for a week or two, and it can be deadly for some. In a typical year, the flu affects between 5% and 20% of the population in the United States. About 36,000 people die from complications of the flu. Flu season can begin as early as October and peak anywhere from late December to early April.

Your best defense against the flu is to get vaccinated each year as soon as the flu vaccine becomes available in your community. The flu vaccine, or flu shot, is given through a needle in your arm. This vaccine contains killed virus. It is approved for all people older than six months.

Flu viruses are divided into three types, designated as A, B and C:

  • Influenza types A and B cause epidemics of respiratory illness almost every winter. They often lead to increased rates of hospitalization and death. Public health efforts to control the impact of influenza focus on these two types.
  • Influenza type C usually causes either a very mild respiratory illness or no symptoms at all. It does not cause epidemics and does not have a severe public health impact.

One reason the flu remains a problem is because the viruses change, or mutate, regularly. This helps the virus to evade the immune system of both children and adults. The process works like this:

  1. A person infected with an influenza virus develops antibodies against that virus.
  2. The virus changes.
  3. The "older" antibodies no longer recognize the "newer" virus when the next flu season comes around.
  4. The person becomes infected again.

The older antibodies can give some protection against getting the flu again. New vaccines are developed each year to protect against the influenza virus strains that are expected to cause the flu that year.

Flu Symptoms

Learn about symptoms that may indicate Influenza.

When to Seek Medical Care

Learn warning signs in children and adults that require urgent medical care.

Who Is at Risk

Certain high-risk patient populations should take special precautions during flu season.

Flu Prevention Strategies

Prevention is the single best way to keep yourself healthy and slow the spread of the influenza virus.

Frequently Asked Questions

Find answers to frequently asked questions about flu season, vaccines, and COVID-19.

Dad with daughter and they are speaking with a physician on their laptop.

Is It Flu or COVID-19?

Symptoms are similar, but treatment options are not. Learn what to do if you get sick and why a flu shot matters more than ever.

Related Resources

Influenza Care Guidelines English | Spanish