Apparently the Dallas ebola patient was not exhibiting any symptoms during his 28 hour journey to Dallas, but it is clear that he was infected prior to his departure from Liberia.
Details of the man's 28-hour trip from disease-ravaged western Africa emerged Wednesday. He flew on two airlines, took three flights, and had lengthy airport layovers - including one at Washington Dulles International Airport - before reaching Texas on Sept. 20.
http://www.wjla.com/...l#ixzz3F0kW0yUg
He did not go to the hospital in Dallas until September 25th (after "falling ill" on the 24th) and it is reported that he had contact with others prior to going to the hospital. He was in contact with hospital personnel prior to being sent home and he was already exhibiting symptoms at that time.
It is not terribly difficult to visualize how an infected individual who has no symptoms (and is from and was infected in the USA) could board a plane in the USA and arrive in Argentina without raising a red flag regarding ebola.