Influenza: The Disease

Influenza Viruses

Influenza, also known as the flu, 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. Influenza 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 the fact sheet "Is it a Cold or Flu?" for more information on flu symptoms.

Anyone Can Get the Flu, But the Disease Is More Severe for Some People

Most people who get influenza will recover in one to two weeks, but some people will develop life-threatening complications (such as pneumonia) as a result of the flu. Millions of people in the United States — about 10% to 20% of U.S. residents — will get influenza each year. An average of about 36,000 people per year in the United States die from influenza, and 114,000 per year have to be admitted to the hospital as a result of influenza. Anyone can get the flu (even healthy people), and serious problems from influenza can happen at any age. People age 65 years and older, people of any age with chronic medical conditions, and very young children are more likely to get complications from influenza. Pneumonia, bronchitis, and sinus and ear infections are three examples of complications from flu. The flu can make chronic health problems worse. For example, people with asthma may experience asthma attacks while they have the flu, and people with chronic congestive heart failure may have worsening of this condition that is triggered by the flu.

For a list of groups that are at high risk for complications from influenza, see: Who Should Get the Flu Vaccine.

The Flu Season

In the Northern hemisphere, winter is the time for flu. In the United States, the flu season can range from November through March, and even past March in some years. During the past 21 flu seasons, months with the heaviest flu activity (peak months) occurred in December in 4 years, January in 5 years, February in 9 years, and March in 3 years.

Peak Months for Flu Activity Over the Past 21 Years

How the Influenza Virus Is Passed Around

The flu is spread, or transmitted, when a person who has the flu coughs, sneezes, or speaks and sends flu virus into the air, and other people inhale the virus. The virus enters the nose, throat, or lungs of a person and begins to multiply, causing symptoms of influenza. Influenza may, less often, be spread when a person touches a surface that has flu viruses on it – a door handle, for instance – and then touches his or her nose or mouth.

The Flu Is 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 persons can be infected with the flu virus but have no symptoms. During this time, those persons can still spread the virus to others.

How To Know if You 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 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 You Should Do If You Get the Flu

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

Influenza 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 an influenza vaccine (flu shot) each fall, before flu season.

Do Not Give Aspirin To a Child or Teenager Who Has the Flu

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 influenza 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.

The Myth of 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 influenza. The flu is a respiratory disease and not a stomach or intestinal disease.

Information provided by the Centers for Disease Control and Prevention