While air travel is a distinct possibility, please remember that illegal immigrants come from all over the globe, and that infection need not occur at the point of origin.
Without playing parsing games, this would be an act of terrorism, not an accidental infection.
In that case, the southern border is a superhighway of invasion.
The ones coming in the airport will have their papers, and likely be either students, government officials, missionaries, or expatriate/visiting workers and will probably not know they are infected..
Without playing parsing games, this would be an act of terrorism, not an accidental infection. While it is possible to think up any number of scenarios by which terrorists would attempt to use Ebola, I will not engage in that exercise. I will continue to limit my comments to my area of expertise, the natural course of infection.
Between all the Americans too'ing and fro'ing between the US and overseas, all the normal, legal, border cross-traffic with Mexico, all the foreigners visiting here on official visas, and legal immigrants here, I feel the problem of Ebola-infected illegal immigrants won't even be noticed. Sure some effort will be made to screen legal travelers for Ebola. The numerical disparity between legal cross-border traffic and illegal traffic is staggering. This is simple numbers.
"The race is not always to the swift, nor victory to the strong. But that's the way to bet." Damon Runyan.