By the Grace of G‑d
3 Iyar, 5715 [April 25, 1955]
Brooklyn, N. Y.
Greeting and Blessing:
I was pleased to read that you have begun your holy work in the field of education...
May the Almighty grant that you not be overwhelmed by the initial difficulties—as per the saying of our sages "All beginnings are difficult"1 —that you will experience. On the contrary: these should arouse in you added vigor and provoke hidden potentials.
Among the advances of medical science in recent times was the discovery of certain vaccines. In order that the body should be able to resist certain aggressive diseases, G‑d forbid, it is injected with a similar and weaker specimen of the same disease; by this means, the body creates weapons and defenses against this disease, which prevent it from ever invading it. The same is true regarding spiritual medicine (as Maimonides, in his famed "Eight Chapters," makes the correlation between physical and spiritual healing). By encountering a weak, so-called "hindrance," the soul creates weapons and defenses with which to vanquish true hindrances and difficulties...
(Freely-translated excerpt from a letter by the Rebbe):2
RE: Spiritual vaccine It seems to me that the Rebbe’s intention here was to teach that the difficulties which his correspondent was experiencing were in truth good, since they helped prepare him for the challenges yet to come. He was comparing this to a vaccine which introduces a harmless form of a virus so that the body will be able to defend itself in the event that the real malady strikes.
So how do you avoid getting sick now? By listening to your doctor and other health professionals—in addition to your Psalms and other prayers.