Two health workers exposed to a patient with the Middle East Respiratory Syndrome (Mers0) have begun showing flu-like symptoms.
One has been hospitalised while the second is being monitored at home, officials at the Dr. P. Phillips Hospital in Orlando, Florida said.
Eighteen more people are now being tested for the deadly disease, which is not thought to be a threat to the general public.
President Barack Obama has been briefed on the two confirmed cases.
A statement from the White House said: "The CDC [US Centers for Disease Control and Prevention] is taking the current situation very seriously."
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A woman hit by police gunfire during a counter-terrorism operation has been arrested after being released from hospital.
Police have been granted extra time to question a man who is being held on suspicion of planning a terror attack.
The man opened fire at the house, shattering the front door and window.