Spiritual health is substantially more nebulous than physical health, and there are many for whom it is so nebulous that they do not ever consider their spirits to have anything to do with life, much less health. Tomorrow is my father's birthday, he would be 89. He was a man of a quiet and very deep faith. I remember him vividly sitting at the table reading his devotions and Bible every morning before going to work. He was a lot like his mother in that he just lived his faith, he didn't push it at you. I remember saying to him once when I was a teenager - "I don't think I believe in God." His words have carried me through four and a half decades since -- "Mark, I don't know that I can prove God exists to you, but I know this much: my life is better for having believed and even if I get to the end of my life and find there was nothing to it, I will still see God as having made me a better person."
Spiritual health can be measured, I think, in terms of how optimistic and trusting you are. One of my favorite definitions of faith is that it is the belief that the universe is kindly intentioned toward you, even when evidence points to the contrary. There are larger dimensions to human existence (and health) than simply skin and bones. In The Message, Eugene Peterson translates Philippians 4:8 this way:
"Summing it all up,friends, I'd say you'll do best by filling your minds and meditating on things true, noble, reputable, authentic, compelling, gracious -- the best, not the worst; the beautiful, not the ugly; things to praise, not things to curse."
There are a host of spiritual values including compassion, humility, forgiveness, patience, gentleness, and kindness -- all of them powerful medicine that heals us individually from the inside, as well as healing for a world that is sick in too many places.