The bird flu spreading through cattle in the United States is an “enormous concern” the chief scientist of the World Health Organization said Thursday as he called for more tracking and preparation for the virus.

So far, there is no evidence that the highly pathogenic H5N1 flu virus can spread from person to person. This flu strain was first detected in birds in 1996 and has primarily been a threat to farmed and wild fowl, but in the past two years, an increasing number of mammals have tested positive with the virus, indicating that the virus is looking for new hosts and moving closer to people.