The U.S. Centers for Disease Control and Prevention says MERS has spread person-to-person but only among close contacts of infected patients. (National Institute for Allergy and Infectious Diseases/Canadian Press)

All hospital workers have tested negative for the MERS respiratory virus at the community hospital in Munster, Ind., where an American is being treated, the hospital's CEO says.

Officials say the man fell ill with Middle East Respiratory Syndrome, or MERS, after flying to the U.S. last month from Saudi Arabia, where he was a health-care worker.

"He has been improving every day," Alan Kumar, chief medical information officer at Community Hospital, told reporters Monday.

"He no longer has any oxygen requirements. He's in good spirits, he's eating well. We have started the discharge planning process. We expect him to be going home soon."

The World Health Organization said Monday that the man is a male U.S. citizen in his 60s who lives and works in Riyadh, Saudi Arabia.

So far, tests on all hospital employees and family members with close contact with the patient have been negative, Kumar said.

"At this point it appears that MERS picked the wrong hospital, the wrong state and the wrong country to try to get a foothold," said Dr. William Van Ness, Indiana's commissioner for health.

The man went went to the emergency room last Monday with a fever, cough and shortness of breath.

Most people who've fallen ill tend to show symptoms within the first five days, the CDC has said. Health officials are  using a 14-day incubation period as a precaution before they consider giving an all-clear that no local spread occurred at the Indiana hospital.

As a precaution, 50 hospital employees who came in contact with the man before he was diagnosed are in isolation at home, Kumar said.

At least 400 cases of the respiratory illness have been reported in the past two years, and more than 100 people have died. Officials say it isn't highly contagious, but there is no cure.

With files from Reuters, The Associated Press and The Canadian Press