All it needs are a few transfers, or connecting flights. It doesn't have to arrive in the same person it left in.
I think that if someone became symptomatic during a flight from an affected country, they would not leave the airport outside of an ambulance. And then they would get better medical care than they could have expected back in Africa, and have a better chance of survival.
I question whether someone seeking to enter the US illegally would have the means to buy an airplane ticket.