The White House announced Tuesday its plan to prevent a bird flu pandemic in the United States. Facts on bird flu:

-- Avian influenza is an infection caused by viruses.

-- There are many different subtypes of type A influenza viruses.

-- There are 16 known HA subtypes and nine known NA subtypes of influenza A viruses.

-- Influenza A (H5N1) virus is an influenza A virus subtype that occurs mainly in birds, is highly contagious among birds, and can be deadly to them

-- H5N1 has caused the largest number of detected cases of severe disease and death in humans

-- There currently is no commercially available vaccine to protect humans against H5N1 virus that is being seen in Asia and Europe.

-- H5N1 virus has crossed the species barrier to infect humans on at least three occasions in recent years

-- Since 1959, instances of human infection with an avian influenza virus have been documented on only 10 occasions

-- The current outbreaks of H5N1 virus began in December 2003 and were first recognized in January 2004

-- 1997: First documented outbreak of human infections with H5N1 in Hong Kong

Sources: CDC, WHO

For complete bird flu coverage, visit FOXNews.com's Bird Flu Center.