Learn how immunizing a critical portion of a community protects most members of the community.
On June 11, 2009, the World Health Organization signaled that a global pandemic of novel influenza A (H1N1) was underway by raising the worldwide pandemic alert level. This action was a reflection of the spread of the new H1N1 virus, not the severity of illness caused by the virus. At the time, more than 70 countries had reported cases of novel influenza A (H1N1) infection and there were ongoing community level outbreaks of novel H1N1 in multiple parts of the world.
When the novel H1N1 flu outbreak was first detected in mid-April 2009, the Centers for Disease Control and Prevention (CDC) began working with states to collect, compile, and analyze information regarding the novel H1N1 outbreak. Read a summary of information gathered during the first weeks of the outbreak. These key disease characteristics are thought to remain an accurate representation of novel H1N1 flu.
Find information from CDC for clinicians and local officials to schools and people at high risk for flu complications and more.
Last Updated December 08, 2009