Flu is caused by influenza viruses that infect the nose, throat, and sometimes the lungs.

The flu virus spreads through tiny droplets released when a sick person talks, coughs, or sneezes. These droplets can enter the mouth or nose of people nearby. Less commonly, a person may get the flu by touching a surface that has the virus on it and then touching their own mouth, nose, or eyes.

People with the flu are most contagious during the first 3 to 4 days after symptoms begin. Some adults may spread the virus for up to 7 days after becoming sick. People with weakened immune systems may spread the virus for a longer period.

Influenza and Older Adults

As people age, their immune systems become weaker. This increases the risk of infections such as the flu. 

Older adults are also more likely to develop severe symptoms compared to younger people. Influenza (the flu) causes about 40,000 deaths each year in the United States. Nearly all of these deaths occur in older adults.

Complications of the flu may include:

• Pneumonia, ranging from mild to severe
• Sinus and ear infections
• Inflammation of organs such as the heart, brain, or muscles
• Multiple organ failure
• Hospitalization or nursing home admission
• Death

People with chronic conditions such as congestive heart failure, asthma, or diabetes are at higher risk of severe illness and may require hospitalization.

Last Updated April 2026