Avian influenza virus, type A strain H7N9, coloured transmission electron micrograph (TEM). This virus first emerged in the human population in China, in March 2013. The virus consists of ribonucleic acid (RNA), surrounded by a nucleocapsid and a lipid envelope (pink). In the envelope are two types of protein spike, haemagglutinin (H) and neuraminidase (N), which determine the strain of virus. The natural hosts of this virus are wild birds. Unusually, infected domestic chickens do not show symptoms, which increases the potential for spread to humans that have contact with chickens. A fifth of human infections are proving to be fatal. As of May 2013 no human to human transmission has been recorded.