The Centers for Disease Control and Prevention (CDC) says it is concerned passengers who were on board two flights the man made may have been exposed to the disease.
He flew into Atlanta, Georgia from Lagos, Nigeria on 9 July, before taking a flight to Dallas, where he was hospitalised, the CDC said.
It said it was working with the airlines to assess "potential risks to those who may have had close contact with the traveller".
A CDC spokesperson told the BBC it was "working with state and local health departments to follow up with individuals who may have been exposed to monkeypox".
"The risk to the general public is thought to be low," the spokesperson said, adding that none of the 200 people they were monitoring were thought to be "high risk".
Though rare, the disease has been discovered in the US before. A 2003 outbreak caused 47 confirmed or probable cases, and was linked to rats flown into the country.