By Nick Carey

CHICAGO, May 17 (Reuters) - An Illinois resident tested positive for Middle East Respiratory Syndrome after being in contact with an infected patient, though he did not show signs of illness, U.S. health officials said on Saturday.

The man likely contracted MERS from a man in Indiana who was hospitalized in late April with the first known U.S. case of the illness.

The Illinois resident's lack of symptoms may shed light on milder forms of the deadly virus, which emerged in the Middle East in 2012 and has infected more than 500 patients in Saudi Arabia alone. It kills about 30 percent of those who are infected.

Researchers at the forefront of the global MERS response said this week they were investigating whether people infected with MERS who have no symptoms could still pass the virus on to others.

"There is evidence there is a broader spectrum of MERS" than first expected, said Dr. David Swerdlow of the U.S. Centers for Disease Control and Prevention, who is leading the U.S. response to MERS.

The Illinois resident did not seek or require medical care and is reported to be feeling well, but officials involved in investigating the first case have been monitoring his health since May 3. A blood test on Friday showed he had developed antibodies to MERS.

CDC officials explained that the blood test is not sufficient to consider him a confirmed case of MERS because it detected only antibodies, not the live virus. Swerdlow said the agency would discuss with the World Health Organization its system of classifying MERS cases to account for milder cases.

On April 25, the Illinois man had a 40-minute face-to-face meeting with the Indiana patient, a business associate, Swerdlow said. The two men shook hands but the Indiana patient did not have a cough at the time.

The Illinois resident has been instructed to avoid other people or wear a face mask. While the Indiana patient was a healthcare worker who had recently arrived in the United States from Saudi Arabia, the Illinois resident had no recent history of travel outside the country, Swerdlow said.

The first case of MERS was confirmed in Indiana in early May and the second, in Florida, on May 11. Swerdlow said 50 people who came into contact with the Indiana patient have tested negative for MERS but are undergoing more tests. Health officials are now trying to identify and monitor close contacts of the Illinois resident.

"It's possible that as the investigation continues, others may also test positive for MERS-CoV infection but not get sick," Swerdlow said.

The disease causes coughing, fever and sometimes fatal pneumonia and reported cases have tripled in the past several weeks. The virus is moving out of the Arabian peninsula as infected individuals travel from the region.

Dutch officials reported their first two cases this week. Infections have also turned up in Britain, Greece, France, Italy, Malaysia and elsewhere.

As MERS is an entirely new virus, there are no drugs to treat it and no vaccines capable of preventing its spread. It is a close cousin of the virus that caused Severe Acute Respiratory Syndrome or SARS, which killed about 800 people worldwide after it first appeared in China in 2002. (Additional reporting by Julie Steenhuysen in Chicago; Editing by Michele Gershberg and Lisa Shumaker)

The Centers for Disease Control and Prevention said Saturday that tests completed Friday show the Illinois man probably was infected with the virus after having close contact twice with the MERS patient in Indiana.

Most cases of MERS have been in Saudi Arabia or elsewhere in the Middle East.