Influenza, or flu, is a contagious respiratory infection caused by influenza viruses. Flu viruses spread from person to person through coughing or sneezing by people infected with influenza. It can also be spread when an infected person speaks, releasing germs into the air that are inhaled by a healthy person, and when a person touches a surface, such as a door handle, that has the virus on it and then touches his or her nose or mouth.
Different strains of the flu virus circulate each year. A new flu virus, the 2009 H1N1 flu, emerged in the spring of 2009 and is spread the same way as seasonal flu viruses. The 2009 H1n1 flu was declared a worldwide pandemic - a global outbreak of serious illness that spreads easily from person to person - on June 11, 2009.
Flu symptoms may include: Fever (100 degrees F or higher), headache, extreme tiredness, dry cough, sore throat, runny or stuffy nose and muscle aches. With H1N1 flu infection, vomiting and diarrhea may also occur.
Adults may be contagious with the flu from 1 day before feeling sick to up to 7 days after getting sick. Children can be contagious for longer than 7 days.
In general, if you get sick with flu-like symptoms, stay home and avoid contact with other people except to get medical care. Most people with 2009 H1N1 flu and seasonal flu have had mild illness and have not needed medical care or antiviral drugs.
However, some people are more likely to get complications from the flu and they should talk to a health care provider about whether they need to be examined if they get flu-like symptoms. Click here for a complete list of people at high risk for developing flu-related complications.
If your flu symptoms are unusually severe, you should go to the emergency room. You should NOT go to the emergency room if you are only mildly ill.
Emergency warning signs:
Children: Fast breathing or trouble breathing; bluish skin color; not drinking enough fluids; not waking up or not interacting; being so irritable that the child does not want to be held; flu-like symptoms improve but then return with fever and worse cough; and fever with a rash.
Adults: Difficulty breathing or shortness of breath; pain or pressure in the chest or abdomen; sudden dizziness; confusion; and severe or persistent vomiting.
The term "flu" is often mistakenly used to describe any mild illness, such as a stomach illness or cold. Several other respiratory viruses circulate during the flu season and can cause symptoms and illness similar to those seen with flu infection. In general, however, the flu is worse than the common cold because the symptoms - fever, body ache, extreme tiredness and dry cough - are more intense with the flu and last longer. People with colds are more likely to have a runny or stuffy nose. Also, colds generally do not result in serious health problems, such as pneumonia, bacterial infections, or hospitalizations.