It was during an ongoing investigation on the first case of Middle East Respiratory Syndrome Coronavirus in the United States that officials identified the new case, the Centers for Disease Control and Prevention said Saturday.
"CDC officials explained that these laboratory test results are preliminary and suggest that the Illinois resident probably got the virus from the Indiana patient and the person's body developed antibodies to fight the virus," the agency said in a statement.
It said the Illinois resident, who has not recently traveled outside the United States, met twice with the Indiana patient before he was identified as the first known case of MERS in the United States.
As part of the investigation, health officials have tested people who came into contact with the Indiana resident.
The identities of the MERS patients have not been released.
The Illinois resident was first tested for MERS on May 5, and those test results were negative. But a blood sample tested positive on Friday, showing he has antibodies to MERS.
"This latest development does not change CDC's current recommendations to prevent the spread of MERS-CoV," said David Swerdlow, who is leading the agency's MERS response.
"It's possible that as the investigation continues, others may also test positive for MERS-CoV infection but not get sick."
The United States has previously announced two confirmed cases of MERS, or Middle East Respiratory Syndrome, which originated in Saudi Arabia and has since spread to more than a dozen countries.
The first patient, who fell ill in April, has been discharged from a hospital in Indiana.
Symptoms of MERS can include fever, chills, cough and in serious cases, kidney failure. Health authorities say it is transmissible mainly through close person-to-person contact and in health care settings.
Meanwhile, health authorities in Saudi Arabia reported three more fatalities from the MERS respiratory virus, taking the death toll in the world's worst-hit country to 163.
The health ministry website also revealed that 520 cases have been recorded in the country since MERS first appeared in Saudi Arabia in 2012.
Egypt, Jordan, Lebanon, the Netherlands, the United Arab Emirates and the United States have also recorded cases, mostly in people who had been to the desert kingdom.
MERS is considered a deadlier but less transmissible cousin of the SARS virus that appeared in Asia in 2003 and infected 8,273 people, nine percent of whom died.
Like SARS, it appears to cause a lung infection, with patients suffering coughing, breathing difficulties and a temperature. But MERS differs in that it also causes rapid kidney failure.