The feds can keep people out of the country, but the US doesn't have the authority to isolate or quarantine other countries, as you suggested.
My point is that even when stuff turns into a crap sammich, the rule of law still needs to apply.
/johnny
Of course if things get really, really bad, history tells us the government can and will suspend the law. I afraid you and I both could be disappointed all too soon. There is an unease in this country. It is an uneasiness about the federal government and I believe well deserved. In the end all government is, is raw force.