Well, they bring-in tuberculosis, malaria, hepatitis, polio, measles, mumps, HIV, syphilis, gonorrhea and other sexually transmitted diseases. We’d eliminated most of those years and decades ago.
I wonder if Central America is like Mexico, in that antibiotics are available over the counter, and people take them until they feel better, but not the whole course of treatment, which would kill all of the pathogens. God knows what kind of drug-resistant superbug might be lurking in the caravan.