Cut-away illustration of an influenza (flu) virus particle (virion). In each particle's lipid envelope (blue) are two types of protein spike, haemagglutinin (H, blue) and neuraminidase (N, yellow) that determine the strain of virus and are used for recognising and binding to the host cell. Spanning the capsid (purple) and envelope are M2 proteins (purple), that act as proton pumps. At the virus' core is the RNA (ribonucleic acid) genetic material complexed with nucleocapsid proteins (red).