Influenza virus. Coloured transmission electron micrograph (TEM) of a single Beijing influenza virus. The virus consists of a core of ribonucleic acid (RNA, red) surrounded by a spiked protein envelope of haemagglutinin (brown). The spikes adhere to host cells during infection. The virus has an affinity for mucus, and is transmitted by coughed or sneezed droplets inhaled into the respiratory tract. Influenza infection is usually short-lived. The haemagglutinin spikes of the virus, however, can change structure to create a new influenza strain such as this Beijing strain which spread as an epidemic in 1993. Magnification: x135,000 at 6x4.5cm size.