3D illustration of influenza virus particle structure. The virus consists of a ribonucleic acid (RNA, orange coils) core, surrounded by a nucleocapsid (purple) and a lipid envelope (yellow). Spanning the capsid and envelope are M2 proteins (yellow), that act as proton pumps. In the envelope are two types of protein spike, hemagglutinin (H, dark yellow) and neuraminidase (N, blue), which determine the strain of virus.