甲型流感病毒是流感病毒分类的一种，包含多种亚型，可感染野生鸟类、驯养家禽、猪、马和人等哺乳类动物。

甲型流感病毒粒子呈球形或呈丝状，直径约80-120nm，有囊膜。甲型流感病毒结构分三层，最外层为双层类脂囊膜；中间层为基质蛋白M1，形成一个球形的蛋白壳；内部为核衣壳，呈螺旋对称，其中包含核蛋白、两种多聚酶蛋白和病毒基因组单链RNA。病毒表面有三种蛋白突起，分别为血球凝集素（H），其作用就像一把钥匙，帮助病毒打开宿主细胞的大门；神经氨酸甘酶（N）能够破坏细胞的受体，使病毒在宿主体内自由传播。而甲型流感病毒亚型的分类命名即以此两种命名。其中N共有16种,H有9种来进行排列组合，而单数的亚型传染力又更强。

Influenza A virus causes influenza in birds and some mammals, and is the only species of influenza virus A. Influenza virus A is a genus of the Orthomyxoviridae family of viruses. Strains of all subtypes of influenza A virus have been isolated from wild birds, although disease is uncommon. Some isolates of influenza A virus cause severe disease both in domestic poultry and, rarely, in humans. Occasionally, viruses are transmitted from wild aquatic birds to domestic poultry, and this may cause an outbreak or give rise to human influenza pandemics.