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.

Centers for Disease Control and Prevention officials told reporters Thursday that there is no evidence that there has been any case of human-to-human spread of H5N1 bird flu in the mysterious situation reported last month in which several people in Missouri suffered illnesses unrelated to known animal contact.