A second health worker who cared for a person hospitalized in Missouri with H5N1 bird flu developed mild respiratory symptoms but was not tested for influenza, the CDC reported.

The Centers for Disease Control and Prevention (CDC) on September 13 shared new information from Missouri officials on recently reported human H5 avian flu case, including that a household contact got sick with similar symptoms the same day and that a healthcare worker had mild symptoms.

In a disclosure that can't eliminate the possibility that bird flu may have spread from one human to another for the first time, U.S. health officials have reported that a person who lived with a Missouri resident infected with H5N1 became sick the same day.

Federal officials are still trying to determine how a Missouri resident with no known exposure to bird flu, contracted the virus.

A second health worker in contact with the Missouri patient who tested positive for bird flu also developed symptoms but wasn’t tested for influenza, the CDC said Friday.

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.

The number of people who were in contact with the hospitalized bird flu patient in Missouri and developed symptoms is now up to three, officials said. All three have recovered. The other two symptomatic cases included a healthcare worker who tested negative for influenza and a household contact of the patient who developed symptoms at the same time as the hospitalized patient,