The Centers for Disease Control and Prevention say the U.S. has seen what is likely the first sign of local transmission of MERS.
The CDC is reporting the first man who brought the Middle East respiratory syndrome coronavirus to the United States seems to have infected someone with whom he had two business meetings.
The unidentified second man lives in Illinois; he had minor symptoms of illness in early May but has recovered.
Blood testing shows the Illinois man developed antibodies to MERS, which is a sign he was infected with the virus.
But he isn't currently considered a case because the World Health Organization case definition for MERS requires a positive test for virus, not antibodies.
The CDC says it will be discussing the case definition with the WHO.
The two men met on two occasions before it was realized that the initial patient was infected with MERS. The first meeting lasted between 30 and 40 minutes, and the second was described as short.
Dr. David Swerdlow, the CDC's point person for the MERS coronavirus situation said the men met face to face, within about two metres of one another, and shook hands.
Swerdlow says the finding does not change the CDC's assessment of how easy it is to contract MERS. He says the CDC considers the encounter as fitting the pattern of a close contact.
The initial case is a doctor and a U.S. citizen who lives and works in Saudi Arabia; the hospital where he works has treated MERS patients.
He developed symptoms of MERS while en route to the United States and sought medical care in Indiana, where he has relatives.
A week after the first case was reported, the U.S. detected a second imported case of MERS, in Florida. The second patient was a doctor as well; he lives and works in Jidda, Saudi Arabia, but had travelled to Orlando via London, Boston, and Atlanta.
One of the contacts of the Florida case came to Canada, only learning after he arrived that he had been exposed to the MERS virus. The man's health is being monitored and he has been isolating himself in his hotel room. But so far he has tested negative for MERS infection.