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 (IK = Igros Kodesh letter number):
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, 10272, 10409, 10123, 6866). Strengthening one's bitachon itself brings healing more quickly and fully (IK 5730, 7637, 7893, 9368, 10514, 6175); during illness, study Shaar HaBitachon (IK 7887). Don't change doctors unnecessarily (IK 10918), and don't despair — doctors can err (IK 2985).
"Tracht gut vet zein gut": think good and it will be good. Positive thinking brings positive outcomes (IK 6692, 7453, 6737, 6685), and so does positive speech; dwelling on and amplifying the negative during illness works against the recovery (IK 937). Thought, speech and action all play a part in healing (IK 6026). After judgment comes compassion (IK 2835); put life's challenges in proper perspective (IK 6118).
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, 734, 9571). Fears are dispelled by turning the mind to positive things rather than engaging the negative thoughts (IK 5790, 819). Avoid harmful brokenheartedness (IK 1061).
Body and soul are one. Physical and spiritual health are interconnected; the body is a vessel for the soul, and caring for it is part of divine service (IK 2001, 2050, 5160, 6016, 3943, 5097, 8358). Spiritual health in the community brings physical blessing (IK 2634). On eating before davening for health and focus, see IK 1356, 1660, 2178.
Adding in Torah and mitzvos. Increase in Torah, mitzvos, Tehillim and daily study as a channel for healing (IK 2387, 3033, 1409, 9183, 10652, 4542); teshuvah and spreading Chassidus go together with healing (IK 1037, 5970). Illness and convalescence can become a time of inspiring others (IK 3019, 646).
For a child or a relative. Strengthen trust in the blessing for a child's or parent's health, together with medical guidance (IK 1186, 1429, 1352, 2913, 1936, 6892). Kindness and personal attention play a part in healing (IK 9389); on encouraging the ill, see IK 2522.
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, 1579).
See also: Health – Following the doctor's advice; Health – Anxiety, depression, nerves and sleep; Health – Checking tefillin and mezuzos; Bitachon – What trust in G-d means: think good and it will be good.