H1N1 flu is like the seasonal flu in that both infections are caused by viruses. H1N1 flu is more common in people younger than age 25. Seasonal flu tends to infect people 65 years of age and older. H1N1 flu also is active earlier in the year. It began to appear in the spring of 2009. Seasonal flu occurs later in the winter, with December, January and February being peak months.

H1N1 influenza (also known as swine flu) is an infection caused by a virus. H1N1 flu began to occur in the United States in spring 2009. At first, the infection was called swine flu because early tests showed that the virus was like flu viruses that occur in swine (pigs) in North America. Since then, further tests have shown that the H1N1 virus is related to 2 flu viruses that occur in pigs in Europe and Asia, 1 that affects birds, and 1 that has been seen in humans.H1N1 flu is like the seasonal flu in that both infections are caused by viruses.H1N1 flu is more common in people younger than age 25. Seasonal flu tends to infect people 65 years of age and older. Symptoms start 3 to 5 days after you have been exposed to the virus and last about 8 days. Symptoms may include:

• Fever
• Sore throat
• Cough
• Muscle aches
• Headache
• Chills
• Fatigue
• Runny or stuffy nose
• Vomiting
• Diarrhea

The H1N1 vaccine is included in the flu shot for the 2010-2011 flu season. The vaccine also protects against 2 strains of the seasonal flu that are expected to be the most common this flu season.

The Center for Disease Control and Prevention recommends that anyone 6 months or older receive the flu vaccine, but especially for groups at higher risk. These include:

• Pregnant women
• People who live with or take care of children younger than 6 months old
• People who work in health care or emergency services
• Children and young adults between 6 months and 24 years of age
• People between 25 and 64 years of age who are at risk of having complications from the H1N1 virus, such as people who have weak immune systems or chronic health problems like asthma or heart disease