Computer illustration of an influenza (flu) virus particle (virion). In each particle's lipid envelope (pink) are two types of protein spike, haemagglutinin (H, ending in three spheres) and neuraminidase (N, ending in four spheres). These determine the strain of virus and are used for recognising and binding to the host cell. At the virus' core is the RNA (ribonucleic acid) genetic material (not seen).