The following is a freely-translated excerpt from a letter by the Rebbe dated Iyar 3, 5715 (April 25, 1955):
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”—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…
. Igrot Kodesh, vol. XI, p. 58.
. Mechilta on Exodus 19:5.