And type A's can be divided into further sub-categories, depending on the subtypes of two proteins present on their surface: hemagglutinin, 'H', and neuraminidase, 'N'.

A型流感病毒还可以进一步细分，取决其表面存在的两种蛋白质的亚型：血凝素(H)氨酸苷酶(N)。