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.

In September, Missouri public health officials and the CDC began investigating the case of H5N1 in a human with no known animal exposure. Out of an abundance of caution, blood samples were taken from five healthcare workers and two household contacts of the infected person who had all displayed symptoms that could have been associated with bird flu.