More than 200 people in 27 US states are being tracked for possible rare monkeypox infections, health officials have said.
The man - believed to be the first monkeypox case in the country since 2003 - was taken to hospital but in a stable condition.
No new cases have been found so far.
The Centres for Disease Control and Prevention (CDC) said 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".
But the chances of the disease spreading on the plane were low because passengers had to wear face masks, it added.