Coloured transmission electron micrograph (TEM) of influenza B virus particles (virions). In each particle's lipid envelope (yellow) are two types of protein spike (green), haemagglutinin (H) and neuraminidase (N). 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 (red). There are three influenza types (A, B and C). Influenza A and B viruses cause seasonal epidemics of disease almost every winter. Influenza A infects human and animals, while influenza B infects only humans.