Seasonal influenza, or flu, is a highly contagious respiratory disease caused by any of several human influenza viruses that circulate globally and cause annual outbreaks of varying severity. Influenza viruses infect the nose, throat, and lungs and produce symptoms that include sudden fever, extreme fatigue, coughing, chills, and muscle aches. Serious complications include pneumonia. Each year, flu causes millions of illnesses worldwide and kills many thousands. It is especially dangerous for people who are very young or old or who have other conditions such as heart disease or asthma.
Pandemic influenza occurs when a new, non-human flu virus emerges from an avian or other animal source with the capacity to spread readily from person to person. Because such viruses have not previously circulated in people, there is no pre-existing immunity to them, and they can cause worldwide (pandemic) disease outbreaks. The most recent influenza pandemic occurred in 2009.
NIAID conducts and supports research to find new and improved ways to diagnose, treat, and prevent flu. This includes research and clinical trials of candidate vaccines that can provide long-lasting protection against both seasonal flu viruses and those with pandemic potential.
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To learn about risk factors for the flu and current prevention and treatment strategies visit the MedlinePlus flu site.