it's more like between the states, not between the US and other countries. One should think of the US analogous to the EU and the state analgous to the countri

it's more like between the states, not between the US and other countries. One should think of the US analogous to the EU and the state analgous to the countries within the EU, with schengen. The virus need only make a beachhead early on (which it did) and then it was inside of an unsealable domain that is wide enough to basically keep things going in perpetuity.