A viral disease
Influenza, commonly called "the flu," is caused by viruses that infect the nose, lungs and windpipe. New strains appear constantly, spreading during the flu season that runs from November through May. Each year, influenza strikes 10% to 20% of the US population, killing 36,000 people annually and sending many more to the hospital.
The best way to prevent influenza is to get a yearly flu shot. Flu viruses change over time, and the vaccine is updated each year to include the most common and potent strains. Flu shots are 70% to 80% effective in preventing influenza in healthy adults. The best treatment is to get plenty of rest and drink lots of liquids.
One to five days after infection, sufferers may develop fever, headache, muscle aches, fatigue, cough, sore throat and runny or stuffy nose. Most people recover completely in one to two weeks, but some can develop life-threatening complications such as pneumonia. Influenza is not related to the "stomach flu," a common digestive illness that causes vomiting and diarrhea.
Influenza is highly contagious. It spreads through droplets in the air, most often when an infected person sneezes, coughs or speaks. Germs can also linger on surfaces.