On Marginal Revolution Tyler Cowen, the economist, says that  "Even a completed fence would not stop a virus..." In fact, diseases require vectors—the viruses are not hiking through the Sonoran Desert, wearing little tiny backpacks, carrying little tiny waterbottles, and singing Mexicanos, al grito de guerra in little tiny voices that you can't hear because they're only  200 nanometers tall.

The viruses come in with people, and an unguarded border that lets millions enter without inspection every year isn't helping.