A close contact of a Missouri resident identified as having a human case of the H5N1 avian flu also became sick around the same time, the Centers for Disease Control and Prevention has revealed.

A patient in Missouri who was hospitalized after an infection with bird flu had the H5N1 strain of the virus, the US Centers for Disease Control and Prevention confirmed Friday.

Epidemiologists have not yet identified exactly how a person in Missouri contracted bird flu last week, said the U.S. Centers for Disease Control and Prevention on Thursday. The human case was the 14th diagnosed in the U.

As bird flu continues to spread through dairy cow herds and poultry flocks, the Missouri case has raised the possibility of human-to-human transmission of bird flu. But officials stressed Thursday there is no evidence of other people being infected and the risk to the general public remains low.

The Centers for Disease Control and Prevention has picked five lab partners to develop a test for avian influenza A (H5N1) — bird flu — including ARUP Laboratories in Salt Lake City. The other lab partners are Quest Diagnostics, Labcorp, Aegis Sciences and Ginkgo Bioworks.

A person sickened by bird flu in Missouri was probably an isolated case, federal health officials said Thursday, though they are still investigating how the person became infected with no clear source of transmission.

A close contact of someone with bird flu became ill on the same day, the C.D.C. reported. But the second person was not tested, and the cause of the illness is unknown.

Public health officials are still trying to determine how a Missouri resident contracted bird flu without having any contact with infected animals, but said Thursday there is no evidence of human-to-human transmission taking place in small enclaves or in a more widespread manner.

Mystery H5 flu case had a close contact who was ill at same time; CDC hopes blood tests will show if there was a connection.