CHICAGO (REUTERS) - An Illinois man who had tested positive for antibodies to the Middle East Respiratory Syndrome virus in his blood is no longer infectious, state health officials said on Monday.

The case in Illinois was the first direct transmission of the Mers virus on US soil. The two prior cases earlier in the month were both "imported" cases of Mers, brought to the United States by infected travelers from the Middle East, the epicenter of the Mers outbreak.

Florida officials separately said the second patient infected with Mers has now been released from the hospital in Orlando.

Since it was first identified in 2012, Mers has infected more than 500 patients in Saudi Arabia alone. It kills about 30 per cent of the people it infects. The virus causes fever, body aches, cough and sometimes deadly pneumonia.

How Mers is transmitted from person to person is not well understood, but most cases have occurred through close physical contact with an infected person or animal, such as a camel, which is thought to be a reservoir for the virus.

In the case of the Illinois man, US health officials relied on blood tests for signs that he had been infected. The tests showed his immune system had fought off a Mers infection.

The man, who had mild, cold-like symptoms, had two business meetings with the first Mers patient to reach US soil before he sought treatment at a hospital in Munster, Indiana.

The man did not test positive for active infection through sensitive tests of samples from his respiratory tract.

Illinois health officials say a second round of test results from oral and nasal swabs show the Illinois man "is not infectious," Dr LaMar Hasbrouck, director of the Illinois Department of Public Health, said in a statement. "What this means is, although the resident was infected at one time, if he sneezes or coughs, the virus is not in his nose or mouth and therefore cannot be spread to others," Dr Hasbrouck said.

Health officials will continue to follow up with the Illinois man and anyone with whom he had close contact. So far, family members who had close contact with the Illinois resident have all tested negative, but will continue to be monitored.