Terrence Tumpey of the U.S. Centers for Disease Control and Prevention said more than one H7 strain of the flu virus has evolved traits needed to easily infect humans, The Times of London reported Tuesday.

While the H5N1 strain is considered the most deadly strain, it is not yet able to move from person to person. A 2003 case in New York, however, showed the H7N2 virus can replicate in the respiratory tract of mammals. A study with ferrets suggests the strain can be passed from animal to animal, the newspaper said.

Researchers said the findings suggest the virus could be acquiring an ability to bind to sugars on the cells of the human windpipe.