Why not say abroad and not just Open Boarders?
Most young children are vaccinated against measles. But outbreaks still occur in the United States, usually when travelers pick up the virus abroad and then spread it among unvaccinated people here.
Read more: http://uk.businessinsider.com/the-measles-outbreak-that-started-at-disneyland-is-spreading-to-the-general-public-2015-1?r=US#ixzz3RNG6uD00
If Americans got shots for their kids only the foreigners would get sick.