Highly pathogenic avian H5N1 influenza viruses are now enzootic in parts of Southeast Asia and the Middle East. Occasionally, these viruses transmit to humans and cause severe respiratory disease and fatalities. Currently, these viruses are not efficiently transmitted from person to person, although limited human-to-human transmission may have occurred –. A major determinant of influenza virus host range is the viral hemagglutinin (HA) protein: avian virus HA binds preferentially to sialic acid linked to the penultimate galactose residue by an α2,3-linkage (Siaα2,3Gal) –, as found for sialic acid–containing receptors of the epithelial cells in duck intestine , the site of avian influenza virus replication. By contrast, human virus HA has higher affinity for Siaα2,6Gal –, the main sialyloligosaccharide on the epithelial cells of the human upper respiratory tract , .