References – AN 5.53: Factors 
“I have an autoimmune disorder of undiagnosed origin. I was wondering if it was worth this precious time of practice to send mettā and thoughts of healing to the body. If so, how do you recommend approaching it? Also, is this compatible with seeing this ailment as a heavenly messenger (and as a contemplation of the body’s demise)? As symptom management reduces it to a mild physical irritation should I also contemplate unpleasant feelings? Or is it best to just stay grounded in the breath and mettā and not risk proliferation. Much gratitude.” Answered by Ajahn Pasanno. [Sickness] [Healing] [Mindfulness of feeling] [Recollection/Death] [Proliferation] [Goodwill] // [Medicinal requisites]
Reference: Amaravati Chanting Book, p. 55: The Five Recollections.
Suttas: AN 5.53-54: Health is a support to the practice. [Health]