Avian influenza virus, coloured transmission electron micrograph (TEM). This virus was isolated from a tern, a type of seabird. The virus consists of a core of ribonucleic acid (RNA), surrounded by a nucleocapsid and a lipid envelope (outer, dark blue layer). In the envelope are two types of protein, 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, and it is feared that human-to-human transmission may become possible.