Think of immigration like the US is a biological cell

...and there are places and things

BUILD A CELL WALL

But keep in mind the cell wall is not penetrated by just everything!!! It is picky!!! Now and then a virus penetrates injecting some transfer RNA causing the cell to make little copies of the virus until the cell walls rupture releasing many copies of the virus to infect other cells!!! In real life we call this a sickness!!! Let the cell wall do its job and destroy the viruses with antibodies or death will result!!! Let the cell wall vet the substances as it was designed to do!!!

wsswan gets the general idea, but there are many things from both sides that affect each other.

Oh and since cell walls are so common it makes sense. There is a lot of action on country borders to control what goes in and out just like a cell well. So there is usually a cell wall.