Amidst surging respiratory illnesses and previously controlled diseases like tuberculosis making alarming comebacks, a new CDC report provides further evidence bird flu is spreading undetected to humans.

The latest Morbidity and Mortality Weekly Report, whose publication was delayed, details three cases of highly pathogenic avian influenza H5N1 in US veterinarians who work with cattle, with two of the cases lacking a clear source of exposure.

None of the vets experienced any flu-like symptoms, and human-to-human spread is still undetected, but researchers are concerned this ability may only be a few genetic mutations away.