NEW YORK -- Health officials on Thursday said they don't know how a Missouri person caught bird flu but believe it may be a rare instance of a "one-off" standalone illness.

Investigators trying to determine how the person caught the virus have not been able to confirm the exact strain of flu.

Centers for Disease Control and Prevention officials said the risk to the general public remains low. The Missouri case raised questions about the possibility of human-to-human bird flu transmission, but officials said there is no evidence of other people being infected.

"Right now, evidence points to this being a one-off case," said Dr. Nirav Shah, CDC principal deputy director.