Influenza (Flu)

Epidemiology and Informatics

Public Health

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What you need to know

Last Update: 02/02/2023

Influenza (flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, including hospitalization and death. 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.

Not only does the spread of influenza take its toll on health, but also has a significant economic toll. The most recent US economic costs measured for seasonal influenza is $11.2 billion in total direct and indirect costs.

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Frequently Asked Questions

What is influenza?

Flu, also known as the influenza, is a contagious disease that is caused by the influenza virus. The influenza virus enters through 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. Complications for the flu can be sinus and ear infections, bronchitis, pneumonia, hospitalization and death, especially in high-risk groups. The flu can make chronic health problems worse.

High-risk groups are at increased risk for severe flu complications:

Young children (2 – 5)
Adults older than 65
Pregnant women
High-risk conditions

Asthma
History of heart disease and stroke
Diabetes
HIV/AIDS
Cancer
Chronic kidney disease
People with muscle and lung function disabilities

Racial and ethnic minorities

Non-Hispanic Black
Non-Hispanic American Indian or Alaska Native
Hispanic or Latinos

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?

Flu can be spread without any known symptoms, but people infected with the influenza virus are the most contagious 3 to 4 days after symptoms appear. Like COVID-19, healthy adults can spread influenza at least 1 day before symptoms appear and 5 – 7 days after flu symptoms. In fact, people with weakened immune systems and young children may spread the flu beyond the 5 – 7 days showing symptoms.

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, respiratory symptoms, and your illness occurs during November through April (the usual flu season in the Northern Hemisphere). It is impossible to tell for sure if you have the flu based on symptoms alone, so it is very important to go and get tested by a doctor within the first few days of your illness because other respiratory illnesses can cause similar symptoms as the flu. The flu is most common between November through April but can be caught at any time of the year."

What should I do if I get the flu?

Stay home and isolate if you are sick, especially if you have a fever
Don't go out and isolate
Rest
Drink plenty of liquids
Avoid using alcohol and tobacco
Take medication to relieve the symptoms of flu

People in high-risk groups with flu-like symptoms should seek medical help to prevent flu complications.

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.