The Rebbe advises the recipient to trust in Hashem, follow medical advice without overanalyzing, and strengthen his connection to Chassidus. He encourages increasing Torah study, fostering a Chassidic atmosphere, and funding a Chassidic sefer, assuring that spiritual efforts will enhance his well-being.
https://www.torahrecordings.com/rebbe/004_igros_kodesh/teves/872