Please! That was the rationale for giving free hospital services to illegals - otherwise they might spread communicable diseases, etc. No matter what you come up with, the government can turn it into something weird. However, I think many states and counties are beginning to take a second look at this type of thing (Prop 200, for example), because it's hitting them so hard in the pocketbook.
I say treat them - and charge Mexico. And if (when) Mexico doesn't pay, deduct it from some of the trade or benefits Mexico gets and give the money to the state or county. Or make Mexico set up some kind of health-care fund in this country for dealing with its nationals.
I was thinking more in terms of using it to deny them entrance. In other words, if they're not healthy, we don't want them. Didn't that used to be the case? And I think that even guest workers should have to have a sponsor who is willing to be responsible for them during the time they are here. That would put a little much needed heat on those who employ them.