So far, patients infected with the deadly H5N1 bird flu virus have rarely passed the virus to others. The molecular explanation for this observation hinges on the anatomical distribution of two closely related sugar structures found on the surface of cells that line the human respiratory tract, according to a team led by Yoshihiro Kawaoka of the University of Wisconsin, Madison (Nature 2006, 440, 435). Influenza viruses bind to host cell receptors that contain complex carbohydrates tipped with sialic acid. Avian flu viruses prefer to bind to receptors that display sialic acid linked to galactose by an α-2,3 linkage (shown), whereas human flu viruses prefer the α-2,6-linked version. Kawaoka's team used fluorescently labeled lectins specific for these two linkages to study the sugars' anatomical distribution in the human respiratory tract. They conclude that the bird virus tends to bind to cells deep in the lungs, whereas human viruses prefer to bind cells higher up in the airway, from which they readily can be spread by sneezing and coughing. A Dutch team has reported similar findings using a more direct technique (Science, published online March 23, dx.doi.org/10.1126/science.1125548).