H5N1 is a highly pathogenic avian infl uenza virus which has caused a widespread epizootic illness among birds. Although the virus is widely present in birds in various parts of the world, human disease from H5N1 has been uncommon as bird to human transmission is ineffi cient and human to human transmission is rare. The virus could cause an infl uenza pandemic if it attains the ability for effi cient and sustained human to human transmission. This is of great concern as there is little natural immunity to H5N1 virus in humans and the disease in humans is severe with a high mortality.