The following is a freely adapted excerpt from a letter written by the sixth Rebbe of Chabad-Lubavitch, Rabbi Yosef Yitzchak Schneersohn, while he was undergoing medical treatment at the Purkesdorf Sanatorium near Vienna in the spring of 1935:

It was a source of much pleasure to me today when I experienced something which can be applied as a lesson in man's service of his Creator.

The doctor came to administer to me a certain treatment, which included the injecting of a healing drug by means of a needle.

I observed the extreme care the doctor and his assistant took in preparing for this. Dressed all in white, they meticulously examined their clothes. They washed their hands twice and three times and checked their nails for the slightest trace of dirt. They poured upon their fingertips a generous amount of disinfectant. They then set to scrubbing my leg with this strong chemical, despite the fact that I had only just bathed.

When I asked about these extensive preparations, they explained that it is an ironclad rule in medicine that before a needle penetrates living flesh one must first go to great extremes to ensure that the entire area is utterly free of the slightest contamination. For were the minutest particle to enter the body together with the healing drug, not only would this cancel the benefit of the medicine, it could also cause many severe illnesses, G‑d forbid.

A Chassidic gathering often involves the injection of a medicine into the body with the prick of a needle. Chassidim rebuke one another regarding their character and behavior. These rebukes, though motivated by an inner love and a sense of concern for the spiritual health of one's fellow, often come in the form of a pricking needle — much the same way that an injection of physical medicine, administered for positive ends and with the best of intentions, must often be accomplished by means of a prick.

But before the needle pricks living flesh, one must ensure that the needle, the hands of the injector, and the area of the injection, are all free of the most microscopic bit of foreign matter. With the neglect of this pre-condition, not only will the "remedy" be rendered utterly useless, but one endangers the very life of the patient, G‑d forbid. For so long as this "contamination" remains outside, it can be eliminated or, at least, swept away; but should it enter within, G‑d forbid, it inflicts great damage.

A gathering of chassidim ("chassidishe farbrengen") is a healing balm, a literal life saver, bringing unimaginable benefit. We have seen time and again how every Chassidic word penetrates to the innermost parts of the mind and heart, how every note of a Chassidic melody awakens the heart, brings it closer to goodness and cleaves it to the truth. But the healing medicines of a farbrengen are administered with a needle, that is, in a tone of rebuke. Therefore, great care must be taken that the "barb" be cleansed and sterilized of the slightest taint of antagonism and self-interest.