Influenza is caused by influenza viruses. It spreads easily from person to person—at home, at school, at work, at the supermarket, on the train. It gets passed on when someone who already has influenza coughs or sneezes. You can also get influenza if you touch something that has the influenza virus—a door handle, a phone, or a pen—then touch your mouth or nose. That's how influenza viruses get into the body.

People can also be infectious (able to pass on viruses) before they even know they have influenza! Healthy adults can transmit influenza from one day before becoming sick and for up to five days after.