1. Seasonal Influenza is a respiratory illness that can be transmitted person to person. Three types of influenza virus typically cause seasonal flu in people. Because the flu virus changes from year to year, vaccination is recommended each year with a vaccine designed to protect against each of the three types of flu expected to cause human illness that year.
2. Seasonal influenza, commonly referred to as the flu, is an infectious disease. In the United States, flu season usually occurs between December and March. The influenza virus is one that has the ability to change easily; however, there is usually enough similarity in the virus from one year to the next that the general population is partially immune from previous infection or vaccination. Each year experts monitor the influenza virus and create a new vaccine to address changes in the virus. For this reason people are encouraged to get a flu shot each year. Also see influenza, avian influenza, and pandemic influenza.
(2006, Pandemic Influenza Business Continuity Guide & Template for San Francisco Businesses. San Francisco Department of Public Health. Communicable Disease Control and Prevention Section.)