Influenza, often simply called flu, is a potentially serious infectious disease that spreads on a seasonal basis. The flu is so common that the exact number of annual infections is not known. According to the Centers for Disease Control and Prevention, an estimated 35 million Americans had the flu in 2019-2020. Of those infected, an estimated 380,000 were hospitalized and 20,000 died. 
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What is Flu?

Flu is a virus that causes flu attacks the respiratory system — the nose, throat and lungs. After the virus enters the body, it begins to multiply inside cells lining the airways. The immune system tries to fight off the infection, a defensive response that produces most flu symptoms. 

Sometimes the flu can make people extremely sick. An out-of-control immune response can lead to significant tissue damage in the lungs. Also, an immune system overwhelmed by flu infection is vulnerable to secondary infection, usually by bacteria, which can trigger an extreme inflammatory response that damages multiple organs. 

Causes of flu

The virus that causes flu spreads from person to person through tiny droplets that enter the air when an infected person sneezes or coughs. It is also possible to pick up the virus by touching an infected surface.