Exactly. Just the ease of travel can spread diseases globally, especially airborne diseases.
When it comes to vector-borne diseases, it’s much easier to contain when it’s just one or two here and there. When it’s groups, it gets much harder. Look at the Yellow Fever outbreak of 1783 in Philadelphia, for example (sudden influx of groups from endemic area). Zika became a worry here with the arrival of groups of immigrants from endemic areas, but, so far at least, better contained thanks to modern methods.
Yes, we would still have diseases carried via international travel. Allowing millions to cross our southern border with no health checks and no record of ingress and destination(s) (as opposed to legal travellers) means it’s harder to trace those spreading diseases and contain the spread, so it’s somewhat more problematic.
I think we’re on the same page here?
I think we are; but we still have to figure out how to instruct migrating birds to stay out of the U.S. :-)