The way a very wise person put it to me is that we can get sick or become unhealthy by only two ways: imbibing something from the outside into our physical bodies, for example by eating unhealthily in the long term, or by poison, or absorbing something through the skin, such as too much ultra violet rays hitting a mole on the skin and causing melonoma. Or, of course, an accident.
The second way is by our emotional reactions to an event, real or imagined, which leaves its mark upon our psyche. Such events are often suppressed or repressed but they nevertheless stay. Within the mind-bodies they grow, eventually precipitating out into physicality, first as symptoms and then as a full-blown psycho-somatic illness. For example, it is possible to get sick from continuing thoughts of revenge, resentment or grief.
The message is clear on the psyco-somatic illness side of things: dwell (in the mind) on Life's pleasant things, not on the things you don't like. Habits of the mind play a big part in our overall health.