New Delhi, June 26 -- The first sign came in the milk.

cows with swollen udders, discolored output, and sharp drops in production. Mastitis, the inflammation of mammary tissue, is one of the most common diagnoses in dairy medicine. The standard cultures came back. Staphylococcus, streptococcus, the bacterial pathogens that cause the condition, were not there.

"Mastitis is a classic disease in milk-production animals, and veterinarians were dutifully looking to all the usual suspects for the source, like bacterial pathogens," said Suresh Kuchipudi, chair of the Department of Infectious Diseases and Microbiology at the University of Pittsburgh School of Public Health. "When the real culprit turned out to be bird flu, everyone in the field...