Avian influenza virus, type A strain H5N1, coloured transmission electron micrograph (TEM). This virus was isolated in Vietnam, during the avian flu outbreak in early 2004. The virus consists of ribonucleic acid (RNA), surrounded by a nucleocapsid and a lipid envelope (green). 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, which show few symptoms, however infected domestic birds suffer a 90-100% mortality rate. Humans that have contact with infected birds can become infected, but no human to human transmission has yet been recorded.