They are undocumented workers. Hardchargers who are just here to do for $5/hour the jobs Americans won't do for $15/hr. Unofficial dual-citizens. Mexican colonists. Unrecognised immigrants. Desert nomads. Wandering minstrels. Affordable Human Resources. Subsidized Healthcare Consumers. Oh, I could go on and on.
"Foreign trespassers" is so terribly unsensitive.
And to suggest that they might be carrying diseases long ago defeated in industrialised nations. Why that's just silly. What about all the experimental medical treatment available only in Mexico?
You forgot the sarc tag...
Some freepers might think you were series.
You forgot that they are "just here to pick fruit and mow lawns."