Influenza is a disease of the respiratory system caused by influenza virus, which is a negative sense single-stranded RNA virus of the Orthomyxoviridae family. The virus can be classified into three subgroups, i.e. influenza A, B and C. Influenza B and C are mainly found in humans, while type A can be found in a wide range of warm-blooded animals. Influenza A viruses can be further categorized into several subtypes based on the antigenicity of two surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). Presently, 15 HA and 9 NA subtypes are identified. Different combinations of HA and NA produce different strains of virus that vary in their pathogenicity.

Highly pathogenic avian influenza (HPAI) causes high rates of morbidity and mortality (up to 100%). On the other hand, low pathogenicity avian influenza (LPAI) may cause only mild symptoms, but LPAI strains may mutate into HPAI strains. In general, HPAI strains are confined to subgroups H5 and H7. H7 influenza subtypes occasionally cause infections in humans, but only rarely result in death. In 2003, one death and 100 other human infections were caused by an H7N7 strain in the Netherlands. In 1997, a strain of H5N1 avian influenza was shown to have directly infected 18 people in Hong Kong, contributing to six deaths. Later, a strain of H9N2 likely derived from poultry was shown to directly infect two people in Hong Kong.

Prevention is one of the best ways to reduce the incidence of influenza and vaccination is one option in the fight against this disease. However, the myriad number of poultry raised in modern farms often makes vaccination difficult. In addition, the HA and NA surface proteins change from time to time, making it impossible for a single vaccine to provide life-long protection.