Influenza, also called “the flu,” is caused by the influenza virus. It affects the lungs and can cause symptoms such as fever and trouble breathing. Most people recover from the flu on their own. But, in some people who are older or have long-term medical problems, the flu can be a serious or even deadly illness.
Influenza is a seasonal illness and affects most people during the winter months. In the United States, January and February tend to be the worst months for flu, but people can get the flu as early as October and as late as May. Each year the virus changes slightly, and the resulting different types of viruses are called strains. Some strains cause more serious illness than others. These strains are sometimes given special names, such as “swine flu” (H1N1) or “avian flu” (H5N1 and H7N9).