Two people sharing a home caught the virus without known exposure to animals. More than 30 human cases have been reported in the United States.

The case concerned health officials and experts because human-to-human transmission of the H5N1 strain of avian influenza could mark the start of a new pandemic.

Human bird flu cases in California may be undercounted as farm workers avoid testing due in part to economic concerns, according to a labor union and a state expert.