U.S. cases of the unexplained pneumonia that has sickened travelers worldwide have climbed to 22, and the disease has been transmitted between Americans for the first time, the Centers for Disease Control and Prevention said Friday.

The troubling news came as the World Health Organization's international count of cases of severe acute respiratory syndrome rose to 350 -- a tally that, because of time differences, does not include all American cases. There have been 10 deaths.

Simultaneously, WHO researchers announced they have isolated an infectious agent that may cause the illness, though they do not know its identity. The researchers said they hope to develop a diagnostic test. But those claims were met with some skepticism at the CDC because the organism causing the illness still has not been identified.

"It's not enough to just find [the organism] in one person," said Dr. Julie Gerberding, the CDC's director.

To prove a link between the illness and a virus or bacteria, the organism will have to be found in samples from most of the people who are sick, and not found in people who are not sick, she said, adding: "This is going to take some time."

There were two additional developments Friday.

Teams from the CDC's little-known quarantine division, who have handed out more than 35,000 information cards to travelers who have been in Asia, have taken into isolation five plane or boat passengers who were ill.

And, for the first time in this country, two victims have been identified as the likely cause of infection in others. One may have infected two health care workers, and another may have infected a family member, Gerberding said.

The CDC declined to say where those patients are located. Overall, the Atlanta-based agency said, there are six suspected cases in California; three in Hawaii; two each in North Carolina, New York and Virginia; and one each in Maine, Massachusetts, New Jersey, New Mexico, Rhode Island, Utah and Wisconsin. A suspected case in Tennessee was ruled out.

Also, the CDC for the first time drew a connection between the United States and the ninth floor of Hong Kong's Metropole Hotel, where the outbreak is believed to have originated with a medical professor from China who infected five other guests and a visitor.

Two Americans traveling separately stayed at the hotel in February and this month. The first traveler has been cleared of causing any secondary cases in the United States, the CDC said; the second traveler is still under investigation.