There are major differences between seasonal flu, which occurs every year, and pandemic flu, which occurs only three or four times in a century.
Seasonal flu (also called common flu) is a respiratory illness caused by a flu virus that has previously circulated in the population and is transmitted from person to person. It usually occurs during the winter months. Most people will have some pre-existing immunity to it, and effective vaccines against infection are produced each season as the virus is identified.
Pandemic flu is a global outbreak of a new flu virus that is easily and quickly transmitted from person to person. Most people will not have pre-existing immunity, and a vaccine may not be available for 4 to 6 months after the virus is identified. A moderate pandemic flu causes more widespread and severe illness than seasonal flu. A severe pandemic flu, caused by a more virulent strain of virus, may result in widespread loss of life.
