"For he will deliver you from the snare of the fowler and from the deadly pestilence" (Psalm 91:3).
This entire psalm is an assurance that those who will recognize their indwelling Lord as their Source for all of life will be protected from earthly challenges. A fowler is someone who hunts birds; the fowler within us, I think, represents an ego energy that seeks to kill our developing spiritual awareness, thought by thought. I understand the pestilence to represent the manifestations of that ego energy that appear in our lives through negative thoughts. However, note that it doesn't promise that there will be no fowler, nor pestilence. We are still living human lives, which brings with it experiences of shadow as well as light. The promise is that, as we experience these thoughts and manifestations, we will be 'delivered' by knowing and believing our spiritual Truth. So we do, indeed, have to be careful in our life—careful of the thoughts to which we attach our spiritual power of faith and the choices those thoughts create. That promise is that we don't have to do it under our own, ego-defined power.