It obviously takes some time for the virus to be detected. I don’t know how much and I don’t think they do..so they use different levels of concern to create different levels of observation to full quarantine.
But that’s for a potential contact.
As for incoming people from the affected area..You just don’t know...and yes people lie every day.
That is why we created Ellis island..
My grandparents came through there...They never talked about it...not once..
We need to go back to having an “Ellis Island”. JMHO