Fragments of the virus that causes bird flu, H5N1, were found in roughly one in five pasteurized milk samples across the US, the US Food and Drug Administration announced Thursday. A day later, the agency posted an update with good news,

Unlike pasteurized milk, which undergoes a process that kills harmful bacteria, experts say raw milk can carry pathogens that make you sick.

Bird flu typically spreads among birds, but there have been recent outbreaks among cattle in the U.S., and one Texas man contracted the virus from sick cows. Here’s why so many experts are worried about a possible new pandemic.