Influenza virus. Cut-away animation of an influenza (flu) virus particle (virion). In each particle's lipid envelope (green) are two types of protein spike, haemagglutinin (H, red) and neuraminidase (N, yellow). These determine the strain of virus and are used for recognising and binding to the host cell. Matrix proteins, which give strength and rigidity to the envelope, are purple. At the virus' core is the RNA (ribonucleic acid) genetic material (yellow).