The flu, also known as influenza, is a contagious respiratory illness caused by three types of viruses: influenza A, influenza B and influenza C. Although people can get the flu any time of the year, the flu season in the U.S. lasts from late fall to early spring, with flu levels peaking between December and March. The flu is normally spread through the coughs and sneezes of an infected person. Adults are contagious 1-2 days before getting flu symptoms and up to 7 days after becoming ill. This means that you can spread the flu virus before you even know you are infected. The best way to prevent the flu virus is to get a flu vaccine every year.

The common cold is a viral infectious disease of the upper respiratory tract. Unlike the flu, which is caused by three influenza viruses, about 200 different viruses cause common colds. Cold symptoms can appear less than two days after being exposed to a cold virus. Similar to the flu, people can catch the common cold year-round. However, colds are more likely to spread when the weather gets cold and people spend more time indoors. Being indoors among many people increases the likelihood of being exposed to one of the cold viruses. Additionally, cold viruses are also spread more easily in colder, drier air. Because of these factors, the cold season in the U.S. typically starts in September and lasts through April.