I don't think it is primarily spiritual. I think people build up toxins in their system faster than they are eliminated, the immune system gets weak, and they are exposed to a bug and catch it.
Water fasts will get rid of it. however, if you have a lot of toxin build up, flushing them out also makes you feel sick. just about any cold will be stopped with a three day fast with only water. Usually you'll start to feel better the second day, but often if you quit the fast you will relapse. If you water fast one 24 hour day a week, you'll keep the toxins out and hardly ever get sick. Or if you eat extremely light food and lots of raw veggies all the time. I don't do either, but do them occasionally. I can feel any bug I catch by a slight fever and tired feeling, and usually start a 24 hour fast then, and never get to the stage of feeling sick.