The flu gets its name from the virus that causes it — the influenza virus. It spreads when people cough or sneeze out droplets that are infected with the virus and other people breathe them in. The droplets also can land on things like doorknobs or shopping carts, infecting people who touch them.

Is the Flu Contagious?

The flu is very contagious. People can spread it from a day before they feel sick until their symptoms are gone. This is about 1 week for adults, but it can be longer for young kids.

The flu usually happens in annual epidemics. An epidemic is when an illness spreads quickly and infects lots of people in an area at the same time. Some years the epidemic is more severe and widespread than others. An epidemic that spreads worldwide is called a pandemic. This is far less common. There were three influenza pandemics in the 20th century, and one so far in the 21st century, in 2009 with influenza A (H1N1).

How Is the Flu Diagnosed?

Often, how a child looks is enough for health care providers to diagnose the flu. Kids who have it usually look ill and miserable.

Other infections can cause symptoms similar to the flu. So if health care providers need to be sure that someone has influenza, they might do a test. They'll take a sample of mucus by wiping a long cotton swab inside the nose or throat. Results might be ready quickly, or can take longer if the test is sent to a lab.

Children with the flu should stay home from school and childcare until they feel better. They should go back only when they haven't had a fever for at least 24 hours without using a fever-reducing medicine. Some kids need to stay home longer. Ask the doctor what's best for your child.