A lot of things come to mind when a supposed candidate for high office makes such statements. That this person is also a doctor whom has sworn to the idea of "first do no harm," makes it even more bizarre.
At the end of the day, though, you have to wonder what has become of a nation when men, who are willing to make such statements so casually, are taken seriously by significant numbers of our neighbors as a potential leader in our highest office. I can only fall back on George Carlin's comment on politics "Garbage in Garbage out."
There are people in a lot of places now running for their lives from unimaginable horror. Husbands, wives and children. Undoubtedly those who create this horror feel no restraint in taking whatever advantage of it that they can, of which, of course, planting a few sleeper agents is but one aspect. There is risk then in taking the real refugees in, but there is also risk in not responding to human need when it presents itself. A risk to our sense of humanity and caring for the plight of others. Ignore that too often and you risk becoming the kind of person who would utter such things as Mr. Carson has said. A man who is also simply trying to take advantage of the suffering of others.