The U.S. confirmed the presence of the bird flu in American cow herds. The W.H.O. said there is risk of the disease spreading to other countries.

With little incentive for US farmers to test their cattle, and many undocumented laborers on dairy farms, the full scale of the outbreak is unclear.

Particles of bird flu virus have come up in milk, sour cream and cheese samples. Baby formula has tested negative. See what products are safe.

However, the agency suggested the milk is not infectious after preliminary tests showed pasteurization is working to kill off the flu in samples. “This additional testing did not detect any live, infectious virus.

The U.S. Food and Drug Administration said on Wednesday that preliminary results of additional testing of more dairy products has shown that pasteurization inactivates the bird flu virus. The FDA released results from tests of products including sour cream and cottage cheese,

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.

But what about raw milk and cheese? While maintaining that avian influenza poses a low risk to the general public, health officials again warned on Wednesday about the potential risk of unpasteurized milk.