Influenza, commonly called "the flu," is caused by the influenza virus, which infects the respiratory tract (nose, throat, lungs). Unlike many other viral respiratory infections, such as the common cold, the flu causes severe illness and life-threatening complications in many people.

The main way that influenza viruses are spread is from person to person in respiratory droplets of coughs and sneezes. (This is called "droplet spread.") This can happen when droplets from a cough or sneeze of an infected person are propelled (generally up to 3 feet) through the air and deposited on the mouth or nose of people nearby. Though much less frequent, the viruses also can be spread when a person touches respiratory droplets on another person or an object and then touches their own mouth or nose (or someone else's mouth or nose) before washing their hands.

Influenza is a respiratory illness. Symptoms of flu include fever, headache, extreme tiredness, dry cough, sore throat, runny or stuffy nose, and muscle aches. Children can have additional gastrointestinal symptoms, such as nausea, vomiting, and diarrhea, but these symptoms are uncommon in adults. Although the term "stomach flu" is sometimes used to describe vomiting, nausea, or diarrhea, these illnesses are caused by certain other viruses, bacteria, or possibly parasites, and are rarely related to influenza.

The period when an infected person is contagious depends on the age and health of the person. Studies show that most healthy adults may be able to infect others from 1 day prior to becoming sick and for 5 days after they first develop symptoms. Some young children people with weakened immune systems may be contagious for longer than a week.

The flu and the common cold are both respiratory illnesses but they are caused by different viruses. Because these two types of illnesses have similar flu-like symptoms, it can be difficult to tell the difference between them based on symptoms alone. In general, the flu is worse than the common cold, and symptoms such as fever, body aches, extreme tiredness, and dry cough are more common and intense. Colds are usually milder than the flu. People with colds are more likely to have a runny or stuffy nose. Colds generally do not result in serious health problems, such as pneumonia, bacterial infections, or hospitalizations.

Because colds and flu share many symptoms, it can be difficult (or even impossible) to tell the difference between them based on symptoms alone. Special tests that usually must be done within the first few days of illness can be carried out, when needed to tell if a person has the flu.

In general, the flu is worse than the common cold, and symptoms such as fever, body aches, extreme tiredness, and dry cough are more common and intense. Colds are usually milder than the flu. People with colds are more likely to have a runny or stuffy nose. Colds generally do not result in serious health problems, such as pneumonia, bacterial infections, or hospitalizations.

Influenza antiviral medications are drugs that suppress the ability of influenza viruses to reproduce. When used correctly, they can reduce the duration of symptoms and some complications from influenza virus infection.

Antiviral drugs are used to prevent and treat the flu. Flu vaccine, however, remains the best way to prevent the flu. When used for treatment, antiviral drugs that are approved for use in the United States may reduce symptoms, shorten the time you are sick by 1 or 2 days, and help prevent you from giving flu to others. Antiviral drugs most often are used to help control flu outbreaks in places such as nursing homes or hospitals, where people are at high risk for complications from the flu and are in close contact with each other. Note that none of the antiviral drugs are approved for use in children less than 1 year of age.

The flu is most severe in younger children and children of any age who have an underlying, long-term illness (such as heart or lung disease [like asthma], metabolic disease [like diabetes], kidney disease, a blood disorder, or a weakened immune system). All children 6 months until their 5th birthday should get a flu shot. All children who have an underlying, long-term illness should also get a flu shot. However, children of any age who want to be protected against the flu should be vaccinated. Healthy¹ children 2 years² or older can choose to take the nasal vaccine or the injected vaccine. The nasal vaccine is approved for use in healthy children 2 years and older. It is not approved for children who have an underlying, long-term illness or children with a history of wheezing. FluMist is not approved for use in children younger than 2 years of age. The flu shot is not approved for use in children less than 6 months old. Children between the ages of 6 months or older but less than 9 years old who are being vaccinated for the first time should get 2 doses of influenza vaccine.

¹ "Healthy" indicates persons who do not have an underlying medical condition that predisposes them to influenza complications.
²On September 19, 2007 the U.S. Food and Drug Administration (FDA) approved use of the nasal influenza vaccine LAIV (FluMist®) for healthy children ages 2-4 years old (24-59 months old) without a history of recurrent wheezing, as well as for healthy persons ages 5-49 years who are not pregnant. Previously, approval was for healthy persons ages 5-49 years who are not pregnant.

By far, the single best way to prevent the flu is for individuals, especially people at high risk for serious complications from the flu, to get a vaccination each year. However, there are other good health habits that can help prevent the flu. These are:

• Avoid close contact with people who are sick. When you are sick, keep your distance from others to protect them from getting sick too.
• If possible, stay home from work, school, and errands when you are sick. You will help prevent others from catching your illness.
• Cover your mouth and nose with a tissue when coughing or sneezing. It may prevent those around you from getting sick.
• Washing your hands often will help protect you from germs.
• Avoid touching your eyes, nose or mouth. Germs are often spread when a person touches something that is contaminated with germs and then touches his or her eyes, nose, or mouth.
• Also, antiviral medications may be used to prevent the flu.