A pandemic is a global disease outbreak. A flu pandemic occurs when a new influenza type A virus emerges for which there is little or no immunity in the human population, begins to cause serious illness, and then spreads easily person-to-person worldwide. A pandemic is determined by spread of disease, not by its ability to cause death.
Flu viruses are constantly changing. A global pandemic (worldwide outbreak) can happen if three conditions are met:
The H1N1 (Swine) flu virus met all three conditions of a flu pandemic and caused the first global pandemic in more than 40 years. The H1N1 flu virus caused more illness in the 2009-2010 flu season in young people and pregnant women than is usual for prior flu seasons. Like seasonal flu, illness in people with H1N1 can vary from mild to severe.
In late spring 2009 the World Health Organization (WHO) declared that an H1N1 flu pandemic was underway. The U.S. Public Health Emergency for 2009 H1N1 Influenza expired on June 23, 2010. On August 10, 2010, the WHO declared an end to the 2009 H1N1 pandemic globally. Internationally, the 2009 H1N1 viruses and seasonal viruses are co-circulating in many parts of the world. It is likely that the 2009 H1N1 virus will continue to spread for years to come, like a regular seasonal influenza virus.
The H5N1 (Bird or Avian) flu virus is an influenza A virus subtype that is highly contagious among birds. Humans have little or no immunity to the H5N1 (Bird) flu virus. The majority of confirmed cases have occurred in Asia, Africa, the Pacific, Europe, and the Near East. Currently, the United States has no confirmed human H5N1 (Bird) flu infections, but H5N1 (Bird) flu remains a serious concern with the potential to cause a deadly pandemic. Avoid close contact with birds that may be infected (dead birds) or their surroundings. The WHO is coordinating the global response to human cases of H5N1 (Bird) flu and monitoring the threat of an H5N1 (Bird) flu pandemic.
The Centers for Disease Control recommends a yearly flu vaccination as the first and most important step in protecting against the flu. Everyone 6 months and older should get vaccinated against the flu as soon as vaccine is available in the fall. The U.S. 2010-2011 seasonal influenza vaccine protected against an H3N2 virus, and influenza B virus, and the 2009 H1N1 virus. The same viruses were selected for the 2011-2012 seasonal influenza vaccine.