Bird flu was probably circulating in dairy cows for at least four months before it was confirmed to be the highly pathogenic H5N1 virus, according to a new analysis of genomic data by scientists at the US Department of Agriculture’s Animal Disease Center.

Is it safe to drink milk? Will the virus jump to humans? Answers to questions about bird flu spreading in dairy cattle.

U.S. sellers of raw milk appear undeterred by federal health warnings for consumers to avoid drinking unpasteurized milk in light of a bird flu outbreak that has affected dairy herds in nine states and sickened at least one dairy farm worker.