Swine flu virus particle. Computer artwork of a swine influenza (flu) virus particle. At the core of the virus is RNA (ribonucleic acid, orange) genetic material. This is surrounded by a nucleocapsid and a lipid envelope. In the envelope are two types of protein spike, haemagglutinin (H) and neuraminidase (N), which determine the strain of virus. These are used for recognising and binding to the host cell.