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