Historically, the pathogenicity of avian influenza (AI) viruses has been based on lethality for the major domesticated poultry species, the chicken. All AI viruses are categorised as either low (LP) and high pathogenicity (HP), but, within each category, pathobiological changes vary with host species and virus strain. Typically, AI viruses that are HP produce a similar severe, systemic disease with high mortality in chickens and a similar pathobiology in other galliforme birds, but usually either produce no infection or mild disease in ducks. The newer H5 and the H7 HPAI viruses have shifted to increased virulence for chickens as evidenced by shorter mean death times (MDT). Furthermore, the Asia H5N1 HPAI viruses have changed from producing inconsistent respiratory infections in ducks to some strains being HP (Hong Kong 2002 viruses) with virus in internal organs and brain, and excretion of large virus quantities from respiratory, and to a lesser extent, intestinal tracts. However, the quantities of virus shed per bird are still 1 % of that produced in chickens. With 2003-2004 Asian H5N1 AI viruses, the isolates have been highly lethal for young ducks but this virulence is age dependent. Across all bird species, the ability to produce severe disease and death is associated with high virus replication titres in the host, especially in specific tissues such as brain and heart.