Rambam dealt with the question of miracles directly in six different places amongst four different works, along with scattered insights elsewhere in the context of addressing specific miracles. However, as with many other topics, there are apparent contradictions between the various statements presented by Rambam. In addition, given Rambam’s explicit warning that he will be employing contradictions to hint at esoteric levels of meaning, this complicates matters even further.
It is exceedingly difficult to synthesize Rambam’s various statements concerning miracles, and radically different conclusions have been drawn by different investigators. On the one hand, Rambam, at least exoterically, considered belief in supernatural miracles to be a necessary corollary to belief in creation. At the same time, Rambam made extensive attempts to minimize the supernatural component of Biblical miracles.
Over the last few decades, several studies on this topic have appeared. I will not be discussing the general topic here. Suffice it to say that there are a range of views as to what Rambam's approach actually was. All agree that he generally desired to minimize the supernatural. Some say that he entirely negated the supernatural element of miracles, others say that he just minimized it as much as possible, while others say that he changed his mind on the matter. In this post, I want to focus specifically on Rambam's approach to leprosy.
(I am using this translation of tzara’as for convenience. However it is clear that the tzara’as of the Torah is not leprosy, otherwise known as Hansen’s disease, which probably only reached the Middle East in the last two thousand years. See Joseph Zias, “Lust and Leprosy: Confusion or Correlation?” It has been proposed that the “leprosy” of the Torah is more accurately translated as “mold” – specifically, Stachybotrys sp. See Richard M. Heller, Toni W. Heller, and Jack M. Sasson, “Mold: “Tsara’at,” Leviticus, and the History of a Confusion.”)
In the Guide, Rambam freely refers to leprosy as miraculous:
All agree that leprosy is a punishment for slander. The disease begins in the walls of the houses. If the sinner repents, the objective is attained: if he remains in his disobedience, the disease affects his bed and house furniture: if he still continues to sin, the leprosy attacks his own garments, and then his body. This is a miracle received in our nation by tradition, in the same manner as the effect of the trial of a faithless wife. (Guide III:47)
Can this be reconciled with the view that Rambam was always opposed to the notion of God having to supernaturally intervene? Indeed, some have viewed these cases as proof positive that Rambam did indeed accept supernatural miraculous events.
I would like to propose a different possibility. Maybe, in Rambam’s view, the miracle was not in the event per se, but in it always happening at the appropriate time. In the Kapach translation, it is not called a nes, but an os ve’pele, “a sign and wonder.” This is consistent with Maimonides’ description of it in the Mishneh Torah:
This change which is spoken of in clothing and houses, which the Torah calls tzara’at by way of borrowed terminology, is not from the way of the world, but is a sign and wonder amongst Israel, to warn them against slander. (Mishneh Torah, Laws of Leprosy 16:10)
This certainly sounds somewhat less than supernatural. It is not “davar hanimna be’ha’teva,” something scientifically impossible (the appellation used in the Treatise to describe supernatural as opposed to naturalistic miracles), only “aino miminhago shel olam,” not the way of the world. While Twersky is convinced that this term refers to a supernatural event, this may not necessarily be the case. The term “way of the world” appears in several places in the Mishneh Torah simply in reference to ordinary norms. Rambam condemns mourners who divert from the “way of the world” in displaying excessive grief, but they are not engaged in a supernatural activity! Rambam describes it as being the “way of the world” that the Egyptians were sent to oppress the Israelites, noting that any individual Egyptian had the choice not to do so, but he is not suggesting that such an Egyptian would have been acting supernaturally. And when speaking about the Messianic Era, Rambam states that there will be no change in the “way of the world” or a change in creation. While this may sound as though such a change would be supernatural (since Rambam does accept drastic changes in the social order), the fact it is followed by the alternative of “a change in creation” indicates that a change in the “way of the world” is something less than that.
In the new Hebrew translation of the Guide by Schwartz, it states that leprosy is a miracle that was perpetuated in the nation. This itself might be the basis for classifying it as a miracle, in line with Rambam’s view in the Treatise Concerning the Resurrection of the Dead that a natural event is rated as a miracle if it perpetuates and especially if it occurs at a significant time. Mold and fungus do occur; here they are occurring in conjunction with a skin disease, and after the person has engaged in evil talk. Perhaps in Rambam's view, these are not innately supernatural events, but when they consistently happen to people who have sinned, they are revealed to be miraculous – that is to say, providentially ordained.
There is another much more radical possibility to be considered. This is perhaps hinted at by Rambam’s stress about how the fear of the result was extraordinarily intense and would prevent people from ever putting themselves in such a situation:
The advantage in this belief is clear. Furthermore, leprosy is contagious, and all people flee from it; this is virtually in their nature. (Guide 3:47)
The benefit is in the effect of the belief about the punishment, more than in the effect of the punishment itself. Rambam stresses how people are in such fear of this contagious disease that it is “virtually natural” for them to avoid it. Perhaps, then, Rambam is relegating these wondrous events to the same category as Biblical accounts of God becoming angry: necessary beliefs to promote social order, but not factually true beliefs. This is not at all straightforward, and a little too Straussian for my personal tastes, but it cannot conclusively be ruled out.
 Joseph Heller, “Maimonides’ Theory of Miracles”; Hannah Kasher, “Biblical Miracles and the Universality of Natural Laws: Maimonides’ Three Methods of Harmonization”; Haim Kreisel, “Miracles in Medieval Jewish Philosophy”; Y. Tzvi Langermann, “Maimonides and Miracles: The Growth of a (Dis)Belief”; Alvin J. Reines, “Maimonides’ Concept of Miracles”; Michael Tzvi Nahorai, “The Problem of Miracles for Maimonides.”
 Yitzchak Twersky, “Halachah and Science,” p. 149.
 Laws of Mourning 13:11.
 Laws of Repentance 6:5.
 Laws of Kings 12:1.
 See Guide III:27-28. Yair Lorberbaum argues as such in Bikoret Ha-Aggadah BeMoreh HaNevuchim, p. 216; see note 66 there. See too James Diamond, “Maimonides on Leprosy: Illness as Contemplative Metaphor,” Jewish Quarterly Review, Volume 96, Number 1, Winter 2006, pp. 95-122 and “Maimonides on Leprosy: From Idle Gossip to Heresy,” (Hebrew) in Maimonides: Conservatism, Originality, Revolution, ed., Aviezer Ravitzky, (Zalman Shazar Center for Jewish History: Jerusalem, 2008), vol.2, pp.375-394