So you are on the right track but there is more to it. Gaines county has somewhere around 2000 illegals or authorized immigrants born outside the USA. That is not a lot compared to bigger counties like Harris or Bexar. It is a contributing factor, but not likely the only cause. What they also have is somewhere between 5k and 6k Mennonites - who also don't typically get vaccinations. When you add those 2 groups together you get a large group compared to the county population (roughly 23k). That means about 1/3 of the population (and probably more) is likely unvaccinated.

I'd bet the initial case came across the border, and spread because of the large amount of unvaccinated people in Gaines. So far it is pretty much contained there, but unless people are really vigilant it could spread outside the area.

As for flea markets - the further south you go in Texas the more of a chance you will get some stupid disease we shouldn't have in the USA. The biggest that comes to mind is Tuberculosis. The illegals are more likely to carry that than anything else. When we did foster care we had to do yearly TB tests because so many of the kids were kids of illegals and spread TB all through the foster care system.

BTW - so far I believe it is 2 deaths being attributed to the measles. One of them they can't prove (even though the media and Texas health authorities say it was) because the kid had improper drugs for their pneumonia. I haven't researched the other death yet - it is pretty recent.