Researchers still don’t know how the recent outbreak of bird flu spread to cattle, but the leading theory is that it has to do with milking machines that could be carrying the virus, Jenna Guthmiller, an assistant professor of immunology and microbiology at the University of Colorado School of Medicine, said in the university’s journal. She said high levels of the virus have been found in the cows’ udders, and the infection appears restricted to dairy cows, which furthers this possibility.