Karen Weintraub and Doug Stanglin

USA TODAY

Another patient has turned up at an American hospital with the lethal respiratory virus MERS, the federal Centers for Disease Control and Prevention reported today.

The agency would not identify the man but said he was a 44-year-old health care worker based in Saudi Arabia, which has been the center of the outbreak.

The man flew to the U.S. from Jeddah, Saudi Arabia, on May 1, to visit family in Orlando, traveling through London, Boston and Atlanta on the way. All the passengers on the U.S. legs of the patient's journey are being notified today to look for possible symptoms of the virus, though the risk of transmission is considered very slight.

Tom Frieden, director of the Centers for Disease Control and Prevention, said the disease has mainly been transmitted by close personal contact, such as directly caring for a very ill person — not through casual contact. The public is not believed to be at risk from the virus, he said, but the warnings are being issued "in an abundance of caution."

The virus is formally called MERS-CoV because it's part of the coronavirus family that includes the common cold and severe acute respiratory syndrome (SARS), which killed more than 800 people worldwide in 2003.

The virus was first identified in people in 2012 but has been spreading rapidly since March. To date, there have been 538 lab-confirmed cases of MERS-CoV worldwide, including 145 deaths attributed to it; the vast majority of those cases occurred in Saudi Arabia, CDC officials said.

The patient in Orlando felt ill on the first flight, officials said, but not ill enough to notify a doctor. Early in the hours of May 9, he apparently became concerned that he was still feeling ill, and he went to the emergency room at Dr. P. Phillips Hospital, according to Dain Weister, a Florida Department of Health public information officer.

He identified himself as recently having been in Saudi Arabia, so he was quickly isolated to avoid contaminating other patients, Weister said.

The man is now in stable condition at the hospital. He has never had the serious cough or other extreme symptoms that sometimes accompanies MERS-CoV, so "that's why we believe the risk of transmission is low," Weister said.

At Community Hospital in Munster, Ind., where the first patient was treated in the U.S., all of the people who had direct contact with the patient have received a second negative test for MERS, and none showed evidence of the infection. The employees were cleared to return to work and discontinue home isolation.

Wayne Marasco, an immunologist at the Dana-Farber Cancer Institute and Harvard Medical School, both in Boston, said his biggest concern about MERS-CoV is how little we know about it. It's still not clear, in some instances, where the virus comes from, how it is transmitted or who is most vulnerable.

SARS, a related virus, was relatively easy to stop a decade ago, because it was being transmitted by caged animals in Chinese markets. In one sweeping exercise, the Chinese military destroyed all those animals, Marasco said, and ended the route of transmission from animals to humans.

In the case of MERS-CoV, which is believed to be connected to exposure to camels, ending that animal-to-human transmission will not be so easy, because an unknown but probably high number of camels in Saudi Arabia carry the virus. Camels can't be eliminated and may not be the only route of transmission from animals, said Marasco, who is working to develop a vaccine against the virus.

The real concern is that the virus will mutate to become more easily transmitted from one person to another, Marasco said, though when he tried to tinker with it in his lab, it became less dangerous rather than more.

Overall, he said, he thinks the CDC has been reacting reasonably, and said he thinks it's appropriate to be concerned but not overreact to the virus.

"I'm not very anxious about it, though I would not at all be surprised if we continued to see cases," Marasco said.

Anna Schuchat, director of the National Center for Immunization and Respiratory Diseases, said the "risk to the general public remains very low."

No vaccine exists for the disease. Treatment consists of standard supportive care for a respiratory illness. Officials said people worried about MERS should wash their hands regularly, wipe down potentially infected surfaces with disinfecting agents and avoid others who are sick.