Scientists from the Scripps Research Institute are reporting that it would take just a single mutation in the version of bird flu that has swept through U.S. dairy herds to produce a virus adept at latching on to human cells, a much simpler step than previously imagined.

To date, there have been no documented cases of one human passing avian influenza to another, the Scripps scientists wrote in their paper, which was published Thursday in the journal Science. The mutation they identified would allow the virus to attach to our cells by hitching itself to a protein on their surface, known as the receptor.