Influenza is an old disease but remains vital nowadays. Three types of influenza viruses, namely A, B, C, have been identified; among them influenza A virus has pandemic potential. The first outbreak of human illness due to avian influenza virus (H5N1) occurred in 1997 in Hong Kong with a mortality of 30%. The most recent outbreak of the avian influenza epidemic has been going on in Asian countries since 2003. As of March 2005, 44 incidental human infections and 32 deaths have been documented. Human influenza viruses differ with other avian influenza viruses on the choice of cellular receptors. Avian influenza viruses bind to cell-surface glycoproteins or glycolipids containing terminal sialyl-galactosyl residues linked by 2-3-linkage [Neu5Ac(alpha2-3)Gal], whereas human viruses, including the earliest available isolates from the 1957 and 1968 pandemics, bind to receptors that contain terminal 2-6-linked sialyl-galactosyl moieties [Neu5Ac(alpha2-6)Gal]. Recent evidence suggests that human bronchial ciliated epithelial cells contain Neu5Ac(alpha2-3)Gal and can be infected with avian influenza viruses. Nevertheless, avian influenza viruses can not infect non-ciliated bronchial epithelial cells. Hence, adaptation of the avian influenza virus to nonciliated cells is a prerequisite for a pandemic virus to emerge. Biological behaviors of influenza viruses indicate that once a pandemic virus emerges, isolation is not likely to contain this epidemic. A specific vaccine against the pandemic strain will not be available until 6 to 12 months after the inception of the pandemic. Judicious use of antiviral agents and stringent disease control measures are imperative to decrease the impact of a future pandemic.