Medicine has begun to realize that repeated use of penicillin and other antibiotics can lead to a dangerous situation where bacteria can modify themselves into new resistant strains which cannot be treated by current therapy.

This phenomenon reminds us of the statement in the Talmud,1that there is no comparison between one who reviews a chapter 100 times and the one who reviews the material 101 times. In the positive, as in the negative, constant repetition can lead, not only to a greater quantity, but to a qualitative change in the effects of a particular activity. In the time of the Talmud, when it was customary to study each chapter 100 times, the addition of just one extra time beyond the habitual number, made all the difference. This benefit is derived for us all whenever we extend ourselves just a little beyond what we are accustomed to do.

From a letter of the Rebbe, Erev Lag BaOmer,5729,
printed in Heichal Menachem, Vol. 2, p. 65ff.