MIT researchers have figured out a possible reason why the bird flu virus hasn’t yet been able to cause widespread illness among humans. In their paper that came out yesterday in Nature Biotechnology, Ram Sasisekharan and his team describe how the H5N1 ‘bird flu’ virus could not bind to the umbrella-shaped sugar molecules found on protein receptors in the human upper respiratory tract, while bird flu viruses known to cause human infection could. The H5N1 virus could bind to shorter sugar chains shaped like cones.

Previously, researchers had thought that differences in the type of bonds between the sugar molecules were important for human resistance to avian flu infection, but it turns out the shape of those molecules are important as well. An Emory scientist is quoted in this Wired story as saying: “People hadn’t really thought about the shape of the structures as being so relevant in this regard.”

According to Nature News, Sasisekharan says that an avian viral protein wouldn’t need to change much (just two or three amino-acid changes) in order to bind to our umbrella-shaped sugars. He added that testing the ability of an emerging avian flu strain to bind to these sugars could provide a sort of early warning of the virus’s ability to make a lot of people sick.