A new study shows the bird flu has silently spread from animals to some veterinarians. It echoes two smaller studies that detected evidence of infection in previously undiagnosed farmworkers.

Three dairy veterinarians, including one who worked only in states with no known bird flu outbreaks in cows, had recent, undetected bird flu infections, according to a new study from the Centers for Disease Control and Prevention. The results are based on antibody testing of 150 veterinarians working in 46 U.S. states.