The unnamed staffer tested negative himself, but Gibbs attributed that to the amount of time that had elapsed since the onset of his own symptoms.
The staffer felt ill three days after his arrival in Mexico City, which was April 13th. On the following day (April 17th), he developed a fever.
He returned to the U.S. from Mexico on April 18th. He traveled on a United Airlines flight from Mexico City to Dulles International Airport in Washington, D.C.
Gibbs did not know the flight number. He also did not know if other passengers on the flight had been alerted to the situation but said that he would check.
Obvious word of warning: if you were on a United Airlines flight from Mexico City to Dulles on April 18th, you should be cautious.
All four individuals (staffer, his wife, his son and his nephew) experienced only mild symptoms, were not hospitalized (or even given medication), and have recovered.