Particles of the Middle East respiratory syndrome, MERS, coronavirus that emerged in 2012 are seen in a colorized transmission electron micrograph from the National Institute for Allergy and Infectious Diseases, NIAID. (Reuters/NIAID)

A second case of the deadly respiratory virus known as Middle East respiratory syndrome has emerged in the United States, once again involving a health-care worker who lives in Saudi Arabia and was visiting his family in the United States, health officials said Monday.

The 44-year-old man is in isolation at Dr. P. Phillips Hospital in Orlando, where he arrived on May 8 showing symptoms of the virus. The Centers for Disease Control and Prevention confirmed he had the virus Sunday night, agency Director Thomas Frieden said in a teleconference Monday.

“The patient is in good condition and is improving,” said Antonio Crespo, one of the hospital’s infectious-disease specialists. “We are taking every precaution but believe the risk of transmission from this patient is very low since his symptoms were mild and he was not coughing when he arrived at the hospital.”

The man left Jiddah, Saudi Arabia, on May 1, and flew to London, then on to Boston, Atlanta and Orlando, where he is visiting family. U.S. health officials are attempting to contact more than 500 people in 20 states who may have been exposed to the patient, and international authorities are doing the same overseas.

Officials said the virus does not appear to be transmitted by casual contact but instead requires close contact for transmission. “Our experience with MERS so far suggests that the risk to the general public is extremely low,” Frieden said.

Sixteen personnel at the Orlando hospital are being monitored for symptoms, and family members of the man are staying at home, a hospital spokeswoman said.

MERS, which was first reported in Saudi Arabia in 2012, can cause severe acute respiratory illness with fever, cough and shortness of breath. More than 30 percent of patients who have symptoms of MERS have died.

There is no vaccine or specific antiviral treatment for MERS, which comes from the same family of viruses as the one that caused severe acute respiratory syndrome. SARS killed almost 800 people around the world in 2003. As of Monday, there were 538 MERS cases in at least 12 countries that have been confirmed and reported to the World Health Organization, including 145 deaths. Of those, Saudi Arabia had 450 cases and 112 deaths, officials said.

The new patient worked at a Jiddah hospital that was treating MERS cases. While in Orlando, he did not visit any theme parks in the area, said John H. Armstrong, Florida’s secretary of health.

Monday’s announcement came just days after a health-care worker who was the first confirmed U.S. case of MERS was released from an Indiana hospital Friday. Health officials said he no longer had symptoms, tested negative for the virus and posed no threat to the community. The Indiana and Florida cases are not linked, authorities said.

Experts have worried that the recent increase in cases may be linked to mutations in the virus. But CDC officials said Monday they found no significant changes or mutations in the genetic sequence of the virus in a sample from the Indiana patient. Some of the increase in cases in the Middle East is because of better monitoring and tracking; the CDC and the WHO have teams in Saudi Arabia to help with the investigations.

Airport and U.S. customs staff members are getting refresher training to look for symptoms among travelers, but the CDC is not recommending airport screening at this time, said Anne Schuchat, director of the CDC’s National Center for Immunization and Respiratory Diseases.

Officials don’t know how many U.S. health-care personnel are working in the Middle East, “but we do know it is not rare,” Frieden said.

In the Indiana case, a U.S. citizen who had been living and working in Riyadh, the Saudi capital, flew to Indiana late last month to visit his family. The hospital where he worked in Saudi Arabia had treated some MERS cases, health officials said. The man flew from Riyadh through London to Chicago before boarding a bus to the Highland, Ind., area. He went to Community Hospital in Munster, Ind., on April 28 after becoming sick. He was kept in isolation.

Authorities contacted most of the passengers on the flights and the bus, and none are symptomatic, according to the Indiana health department.

CDC officials praised the Indiana hospital for its aggressive efforts to control the infection. Once the patient was a confirmed MERS case, hospital officials reviewed security tapes, monitored the sign-ins required of everyone — from doctors to housekeepers — who entered the patient’s room and tracked them through their ID badges, embedded with electronic chips, which show their locations at all times. About 50 employees were sent home and kept isolated until the hospital was sure they did not have MERS. They were planning to return to work Monday and Tuesday, according to Alan Kumar, the hospital’s chief information officer.