Because it does not seem to make birds sick, its presence in flocks may go unrecognized until people start to get sick. In addition, Dr. Cox said, it may be difficult to make an effective vaccine. Past efforts to produce vaccines against related viruses were unsuccessful.

Many questions about the virus remain. Researchers are not sure how deadly it is because they do not know how many people have been infected. The worst cases become obvious, but if some people have mild symptoms or none at all, many cases could go undetected.

Richard Webby, of the infectious diseases department at St. Jude Children’s Research Hospital in Memphis, who was not involved in the report from China or the commentary, said in an interview that compared with other types of bird flu, H7N9 was the most worrisome he had seen because it seemed the most capable of infecting humans.

On Thursday, the C.D.C. received its first specimen of the virus from China. It was a sample from one of the patients described in the journal article, a 35-year-old woman from Anhui who died on April 9 after 19 days in the hospital. The sample consisted of a tiny bit of egg white in a vial, containing tens or possibly hundreds of millions of viruses, Michael Shaw, a flu expert at the C.D.C., said in an interview.