Bird flu was detected in a pig for the first time in the U.S. Where? At least one pig at a backyard farm in Oregon was infected with the H5N1 virus that has swept the nation’s poultry and dairy cows, officials said yesterday. Why it’s worrying: Pigs can become coinfected with bird and human viruses. That allows genes to swap to form a new, more dangerous virus that can more easily infect humans.