WHO limits pandemic risk levels to four ; level 4 represents global pandemic . No pandemics have occurred since 1918 . It was sparked by the H2N2 strain and was first identified in China . Spanish flu was the worst pandemic of the 20th century .
WHO limits pandemic risk levels to four ; level 4 represents global pandemic . No pandemics have occurred since 1918 . Spanish flu was the worst pandemic of the 20th century .