Influenza virus particle. Artwork showing the external structure of an influenza virus particle. The outer surface consists of two types of protein embedded in a lipid envelope. These surface proteins are haemagglutinin (H, red) and neuraminidase (N, spike). These surface proteins are used for recognising and binding to the host cell. The proportion of H and N proteins determine the strain of influenza virus and the severity of an infection. Examples (for influenza A, the most common type) are H1N1 (swine flu and Spanish Flu), H5N1 (avian flu), H2N2 (Asian Flu), and H3N2 (Hong Kong Flu).