Influenza (flu) is a contagious respiratory illness caused by viruses. Infection results in mild to severe illness and can lead to death. Every year, an average of 5 to 20 percent of the U.S. population gets the flu. Some influenza viruses can also infect birds, pigs, horses, seals, whales and other animals.

The H1N1 influenza (or swine flu) is a new strain of the influenza virus. Its symptoms are similar to those of the seasonal flu and may include fever, cough, sore throat, body aches, headache, chills, fatigue and the worsening of underlying chronic medical conditions. Some people have also reported diarrhea and vomiting associated with H1N1 flu. Unlike with the seasonal flu, young people are at higher risk of H1N1 infection than people 65 and older. However, infected people 65 or older are still at increased risk of H1N1 influenza-related complications. The worldwide spread of the H1N1 virus this spring was both rapid and unusual as it affected some countries outside of the timeframe of a normal flu season. While most people who have become ill with the H1N1 virus have recovered without needing medical treatment, hospitalizations and deaths have occurred.

The H1N1 flu and seasonal flu spread in much the same way. Flu viruses spread mainly from person to person when people with influenza cough, sneeze, or touch things that others touch. People infected with the H1N1 virus can spread it to others one day before symptoms develop and up to seven or more days after becoming ill. That means that you may be able to pass the flu to someone before you know you are sick and after you start to feel better.