Influenza viruses belong to the family Orthomyxoviridae. The viral particles are about 80-120 nm in diameter and can be spherical or pleomorphic. Their lipid membrane envelope contains two glycoproteins: hemagglutinin (HA) and neuraminidase (NA). These two proteins determine the subtypes of Influenza A virus (i.e. swine flu strain, avian flu/bird flu strains). There are 18 H subtypes and 11 N subtypes.