Influenza (Flu)

Epidemiology and Informatics

Public Health

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What you need to know

Influenza (flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness. Serious flu infections can result in hospitalization or death.

Some people, such as older people, young children, and people with certain health conditions, are at high risk for serious flu complications. The best way to prevent the flu is by getting vaccinated each year.

Flu Prevention and Vaccine Information

Weekly Influenza (Flu) Report

Information for Health Professionals and Labs

Information for Long-Term Care Facilities

Information for Schools

Frequently Asked Questions

What is influenza?

Flu, also known as the influenza, is a contagious disease that is caused by the influenza virus. It attacks the respiratory tract in humans (nose, throat, and lungs). The flu is different from a cold. Flu usually comes on suddenly and may include these symptoms:

Fever
Headache
Tiredness (can be extreme)
Dry cough
Sore throat
Nasal congestion
Body aches

These symptoms are usually referred to as "flu-like symptoms." Please also see Influenza vs. Colds vs. Pertussis for more information.

Who is most at risk for getting very sick from flu?

Anyone can get the flu, even healthy people, and serious problems from the flu can happen at any age. Most people who get flu will recover in one to two weeks, but some people will develop life-threatening complications as a result of the flu.

People 65 years and older, people of any age with chronic medical conditions, and very young children are more likely to get complications from the flu. Pneumonia, bronchitis, and sinus and ear infections are three examples of complications from the flu. The flu can make chronic health problems worse.

For a list of groups who are at high risk for complications from flu, see: Who should get the flu vaccine.

Is the flu contagious?

Yes. Everyone 6 months and older should get flu vaccine every year. Getting a flu vaccine helps protect you from getting the flu and prevents you from passing it to people who could get very sick.

How long is a person with a flu virus contagious?

A person can spread the flu starting one day before he or she feels sick. Adults can continue to pass the flu virus to others for another three to seven days after symptoms start. Children can pass the virus for longer than seven days. Symptoms start one to four days after the virus enters the body. Some people can be infected with the flu virus but have no symptoms. During this time, they can still spread the virus to others.

How do I know if I have the flu?

Your respiratory illness might be the flu if you have sudden onset of body aches, fever, and respiratory symptoms, and your illness occurs during November through April (the usual flu season in the Northern Hemisphere). However, during this time, other respiratory illnesses can cause similar symptoms and the flu can be caught at any time of the year. It is impossible to tell for sure if you have the flu based on symptoms alone. Doctors can perform tests to see if you have the flu if you are in the first few days of your illness.

What should I do if I get the flu?

Rest
Drink plenty of liquids
Avoid using alcohol and tobacco
Take medication to relieve the symptoms of flu

Flu is caused by a virus, so antibiotics (like penicillin) don't work to cure it. The best way to prevent the flu is to get a flu vaccine (flu shot) each fall, before flu season. Never give aspirin to children or teenagers who have flu-like symptoms – and particularly fever – without first speaking to your doctor. Giving aspirin to children and teenagers who have the flu can cause a rare but serious illness called Reye syndrome. Children or teenagers with the flu should get plenty of rest, drink lots of liquids, and take medicines that contain no aspirin to relieve symptoms.

What is the "Stomach Flu"?

Many people use the term "stomach flu" to describe illnesses with nausea, vomiting, or diarrhea. These symptoms can be caused by many different viruses, bacteria, or even parasites. While vomiting, diarrhea, and being nauseous or "sick to your stomach" can sometimes be related to the flu – particularly in children – these problems are rarely the main symptoms of flu. The flu is a respiratory disease and not a stomach or intestinal disease.