An extensive look at wastewater samples taken across the United States from May to July found traces of the H5N1 bird flu popping up -- but only in areas populated by farm animals.

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.

Researchers in Texas detected H5N1 in wastewater from 10 cities, but the risk to humans is currently low. Ongoing surveillance is critical to monitor any potential changes in the virus that could increase its threat to public health.

On Monday state officials announced they had completed testing on all 95 licensed farms in Massachusetts that have dairy herds, the first state in the nation to do so. The results were “100 percent negative.” The testing program is likely to be repeated on a regular basis going forward, and could serve as a national model.

"The widespread detection of influenza A (H5N1) virus in wastewater from 10 U.S. cities is troubling," the authors concluded. "Although the exact origin of the signal is currently unknown, the lack of clinical burden along with genomic information suggests multiple animal sources."