A man (53) is in isolation in a New York hospital after testing positive for bubonic plague.
A woman who was travelling with the man from New Mexico has also been admitted to hospital, but her tests had not been completed.
Officials said there was no evidence of danger to the public.
Bubonic plague is one of a handful of agents the FBI fear could be used in a bioterrorist attack. But officials at Beth Israel Hospital, where the pair were being treated, said the man's case almost certainly was natural.
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The couple showed up at the hospital's casualty department after several days of fever and swollen lymph nodes. About one in seven cases is fatal.
Humans usually contract the disease from fleas that have fed on infected rodents.
AP