The subtypes of hemagglutinin range from 1 to 16, while the neuraminidase subtypes are numbered 1 through 9. The reassortment of hemagglutinin and neuraminidase results in viruses with different degrees of virulence causing different symptoms, and the host organisms they infect also differ.
H 有 1-16 型，N 則有 1-9 型，H 和 N 經過各種不同組合後，所產生的病毒毒性不同、症狀不同、感染的對象也不同。