A business associate of the man who brought the first case of a mysterious Middle East virus to the U.S. has also tested positive for the disease, though he showed no signs of illness, federal health officials said Saturday.

The new infection — the third reported in the U.S. and the first transmitted on American soil — is in an Illinois man who met and shook hands with a health care worker who was hospitalized in Indiana after traveling from Saudi Arabia and was diagnosed May 2 with MERS, or Middle East respiratory syndrome.

The Illinois man had not traveled outside of the U.S. recently and he did not seek or require medical care, officials with the Centers for Disease Control and Prevention said in announcing the new infection Saturday. Instead, laboratory tests showed that the man had evidence of an apparent past infection in his blood. He continues to feel well, CDC officials said.

“This latest development does not change CDC’s current recommendations to prevent the spread of MERS,” said Dr. David Swerdlow, who is leading the agency’s response to the infection that has sickened more than 570 people and killed 172, mostly in Saudi Arabia. It is formally known as the MERS coronavirus, or MERS-CoV.

“It’s possible that as the investigation continues others may also test positive for the MERS-CoV infection but not get sick,” Swerdlow added in a statement.

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Although he has evidence of past infection, the Illinois man is not considered an official MERS case under World Health Organization and CDC defintions, Swerdlow said.

The new infection was detected as part of efforts by CDC and state health departments to contact everyone connected with the Indiana man and a Florida man who was the second in the U.S. diagnosed with MERS May 11.

In both cases, the men were health care workers who came from Saudi Arabia and traveled on planes and other forms of public transportation to get home. The Indiana case is a health care worker in his 60s who was hospitalized April 28 and then diagnosed with MERS. In the Florida case, the 44-year-old man went to an Orlando emergency room, where he may have exposed others to the virus. The U.S. men with MERS infections have not been identified.

MERS is spread through close contact, health officials say, and there’s no evidence of sustained transmission in public settings.

But in the case of the most recent patient, who lives in Cook County, Illinois, according to the CDC, a "close contact" included a 30- to 40-minute, face-to-face business meeting, Swerdlow told reporters Saturday. The two also held another, briefer meeting in the days before the Indiana man tested positive.

That doesn't change how CDC views the possibility of the spread of the disease, Swerdlow said.

"It’s not sustained transmission and it’s not easy transmission," he said.

Local health officials reached out to the Illinois man and tested him for active infection on May 5, with negative results. But blood samples in a test returned Friday, May 16, showed evidence of MERS antibodies, indicating recent infection.

CDC officials are continuing to reach out to, test and monitor people who have come into contact with the three U.S. residents with evidence of infection, Swerdlow said. Health workers will now reach out to the circle of contacts of the Illinois man to look for signs of sickness or evidence of infection. Meanwhile, he has been asked to isolate himself from contact, either by staying home or wearing a mask when he goes out, Swerdlow said.

Officials say they have not changed their travel guidelines for U.S. residents heading to the Middle East and urge people to take sensible infection control precautions. The agency recommends that travelers closely monitor their health during and after the trip and report any signs of illness.

Health officials have been evaluating travelers from the Middle East for months and should continue to do so, with extra vigilance about any signs of respiratory illness.