The U.S. may be missing human cases of bird flu, scientists say

The U.S. Department of Agriculture is ordering dairy producers to test cows that produce milk for infections from highly pathogenic avian influenza (HPAI H5N1) before the animals are transported to a different state following the discovery of the virus in samples of pasteurized milk taken by the Food and Drug Administration. (Justin Sullivan/Getty Images)

Officially, there is only one documented case of bird flu spilling over from cows into humans during the current U.S. outbreak.

But epidemiologist Gregory Gray suspects the true number is higher, based on what he heard from veterinarians, farm owners and the workers themselves as the virus hit their herds in his state.