Bird flu, also known as avian influenza or H5N1, has infected more than 130 million birds over the past two years as it threatened commercial bird operations and recently led to the first human death in the United States.

The patient, who was older than 65 and had underlying medical conditions, was hospitalized weeks ago in critical condition with severe respiratory illness.

A person in Louisiana who was hospitalized with a severe case of bird flu has died. The hospitalization, first announced Dec. 13, marked the first severe case of H5N1 bird flu in the United States. H5N1 is a subtype of bird flu that's been circulating among wild birds and poultry in the U.