Avian influenza may be just “three mutations” away from being transmittable via human-to-human contact, a Cambridge University study revealed. Currently, the H5N1 virus strain can only be contracted from infected birds.

As of this writing, the World Health Organization reported 606 human cases of bird flu since records began in 2003. During that period, the deadly strain killed 357.

Researchers found that the evolution of the virus could occur in a single person, but cannot as of yet accurately predict when this will happen.

A press release from the university quoted Professor Derek Smith, who co-authored the study: “With the information we have, it is impossible to say what the exact risk is of the virus becoming airborne transmissible among humans. However, the results suggest that the remaining three mutations could evolve in a single human host, making a virus evolving in nature a potentially serious threat.”