Correct Option: C

Influenza A virus subtype H5N1, also known as “bird flu”, A(H5N1) or simply H5N1, is a subtype of the influenza A virus which can cause illness in humans and many other animal species. H5N1 isolates found in Hong Kong in 1997 and 2001 were not consistently transmitted efficiently among birds and did not cause significant disease in these animals. In 2002, new isolates of H5N1 were appearing within the bird population of Hong Kong. These new isolates caused acute disease, including severe neurological dysfunction and death in ducks. This was the first reported case of lethal influenza virus infection in wild aquatic birds since 1961.