Influenza virus particle. Cutaway artwork showing the internal and external structure of an influenza virus particle. The internal structure includes coils of RNA (ribonucleic acid) genetic material. This is surrounded by a nucleocapsid and a lipid envelope. The external structure consists of two types of protein embedded in the 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.