What you are missing is that illegals generally travel in groups, and meet up with other groups along the way. Disease does not have to travel all the way here in one host, it can be relayed. The sick person will infect at least some of the other people encountered. If an illegal becomes very ill, they try even harder to get here for medical treatment- meaning they would be unlikely to stop their travel until their illness forces them to stop.
Ebola is only communicable once the person becomes symptomatic. At that point, they become rapidly debilitated, rendering them unable to travel. In the hot climate of Mexico, without medical treatment, their condition would decline very rapidly.
I seem to recall reading that the coyotes—the human traffickers—have no patience with people who can’t keep up with the group, and routinely abandon people to die. For that reason, an Ebola victim would probably be abandoned at the first symptom. The coyotes certainly aren’t going to spend any time trying to care for that person. In that situation, the chance of a second person contracting Ebola is slim.
Also, one thing about highly lethal diseases is that they kill their victims so quickly that the disease has no chance to spread. So, assuming a group of illegals were to stop and try to care for an Ebola victim in their midst, they would all infect themselves and their journey would end there.