Flu seasons are unpredictable in a number of ways, including the timing of the beginning, severity, and length of the flu season.

This flu season (2009-2010), there are more uncertainties than usual because of the emergence of a new 2009 H1N1 (Swine) flu virus.

Severity is uncertain. The 2009 H1N1 flu virus - along with regular seasonal viruses - will cause illness, hospital stays, and deaths this flu season in the United States. Scientists are concerned that the 2009 H1N1 virus may cause the season to be worse than a regular flu season - with a lot more people getting sick, being hospitalized and dying than during a regular flu season.

Timing is uncertain. In past years, seasonal flu activity typically did not reach its peak in the U.S. until January or February, but flu activity has occurred as late as May. However, the 2009 H1N1 flu virus caused illness, hospitalizations, and deaths in the U.S. during the summer months when influenza is very uncommon. So it is not known when flu activity will increase, when it will be most intense (peak), what viruses will circulate, or how long the season might last.