A wakeful consciousness

This is the only true defence: a wakeful consciousness, pure and alert, so to say, which does not sleep, does not let things enter without being aware of them. The worst thing is that people are quite unconscious and that it is only after the contagion has entered that they notice it, and it is a little late to react—it is not impossible, but it is more difficult—while if one sees it coming, if in the surrounding atmosphere it comes making a kind of little black mark, one can chase it off as one would something disagreeable. But the protective envelope on the material plane has no effect in this instance.

Ref: Questions and Answers 1950-1951