These are the Rebbe's letters to individuals about their own situations. Throughout, the Rebbe instructed people to consult an expert doctor and follow his guidance; anyone facing a medical question should do the same.
Alongside following the doctor, the Rebbe's guidance to the sick and their families returns again and again to trust in Hashem:
Hashem is the true Healer, and the doctor is His agent. Trust in Hashem does not replace medical care; the two go together (IK 1071, 1612, 2922). Strengthening one's bitachon itself brings healing more quickly and fully (IK 5730, 7637, 7893, 9368).
"Tracht gut vet zein gut": think good and it will be good. Positive thinking brings positive outcomes (IK 6692, 7453), and so does positive speech; dwelling on and amplifying the negative during illness works against the recovery (IK 937).
Don't dwell on worry or medical reports. Leave the medical details to the professionals, and fill the mind with Torah and one's mission instead (IK 1362, 1619, 1628, 7202). Excessive anxiety about health is itself harmful, and many lingering worries are imagined (IK 1633, 2307, 2903, 3502, 4031). Fears are dispelled by turning the mind to positive things rather than engaging the negative thoughts (IK 5790, 819).
Don't be discouraged by negative medical predictions. Strengthen trust and take the practical steps (IK 5499). Miracles beyond nature happen for individuals today too (IK 2140). Recalling past kindnesses from Hashem puts present difficulties in perspective (IK 4908, 9264).
Rather than relying on amulets, strengthen trust in Hashem as the source of healing (IK 9397). Joy is itself a remedy that opens channels for healing (IK 8235).