Multiple choice The H5N1 virus which causes bird flu was first discovered in ______. 1991 1995 1997 2001 Reveal answer Fill a bubble to check yourself C Correct answer Explanation H5N1 (bird flu) was first discovered in 1997 during an outbreak in Hong Kong. This virus affects birds and can spread to humans. The 1997 outbreak was the first known instance of this highly pathogenic strain.