H5N1 is the most worrying of the 16 sub-types of avian influenza virus haemagglutinin. It mutates quickly and as an added propensity to acquire genes of viruses that infect other species. It has caused serious infections in human beings on two occasions. In addition, laboratory studies have shown that isolates of a virus are highly pathogenic and may cause serious diseases in human beings. Birds that survive this infection excrete the virus for at least 10 days both orally and in faeces that facilitates its spread in live poultry markets and through migrating birds.
The highly pathogenic avian influenza epidemic due to the H5N1 virus which began in mid-December 2003 in the Republic of Korea and which is now seen in other Asian countries raised particular public health concerns. The H5N1 varieties were shown to be directly able to infect human beings in 1997 and this reoccurred in Vietnam in January 2004. Spread of the infection in birds increases the possibility of direct infection in human beings. While the number of cases of human infection is increasing over time, it is increasingly likely that people infected by human and Avian strains will act as a "melting pot" for the development of a new sub-type with sufficient genes from the human virus to be able to transmit the infection between people easily. This would then indicate the beginning of a pandemic.