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(CBS NEWS) For the first time, the respiratory illness MERS has spread from one person to another in the United States.

The Centers for Disease Control and Prevention says an Illinois man got the virus from an Indiana health care worker, who brought it from the Middle East. Their only physical contact was a handshake.

MERS, or Middle East respiratory syndrome, is blamed for nearly 200 deaths worldwide.

The contact is "very concerning," said CBS News' Dr. Holly Phillips, because up to now, the focus of investigators has been on prolonged close contact, meaning people who live with a sick patient or health care workers who have come into contact with a person with MERS.

"In this case we know the two patients met briefly twice," Phillips said. "Their longest meeting was just 40 minutes, and each time they shook hands. So that's not really considered close contact. So what the CDC is going to have to look for is if the virus is simply more contagious than we thought it was before or it's changed in some way. ... Forty minutes together and a handshake is not very, very close contact."

Another twist in this case, is that the person infected with MERS did not exhibit any symptoms, Phillips said.

"We do know up to 20 percent of MERS cases can be asymptomatic or the symptoms are just so mild that people don't notice them," Phillips said. "On one hand, we think that's a good thing, that the virus can be mild. On another, it might allow for it to spread quickly without people even realizing that they're spreading it."

About 30 percent of the documented MERS cases have been fatal, Phillips explained.

"It is a very serious illness," she said. "The two cases here, though, both patients were very sick. They got supportive care. There is no cure or vaccine. They got supportive care in the hospital -- whether it was oxygen or fluids -- and they both turned the corner. So it might be more treatable than we have thought before."

"This is not something to worry about on the day-to-day," Phillips added. "But it's really important that we stay vigilant, and it's great that the CDC and the World Health Organization -- they're both all over it."

SEE ALSO: Florida MERS patient released from hospital

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