The Mishna in Masekhet Negaim (4:5), in discussing the laws of skin tzara’at, addresses the case of two white discolorations which appear on a person’s skin, and they are connected by a thin line of white skin. The question in such a case becomes whether these two spots are to be treated as two separate infections, or as a single infection. The Mishna establishes that in this instance, the two spots are considered a single discoloration, unless the strip of white connecting them is exceptionally thin – not even the width of two hairs.
The Rash Mi-Shantz explains the practical implications of this halakha. If the line connecting the two spots is just a hairsbreadth, then the properties of one do not affect the other. For example, the presence of two white hairs in a suspected tzara’at infection confirms the infection as tzara’at, conferring a status of impurity upon the individual and requiring him to undergo the process outlined in the Torah for metzora (persons stricken with tzara’at). If the line connecting two spots is only a hairsbreadth, then the presence of two white hairs in one area would render that area a definitive tzara’at infection, but not the other. An earlier Mishna (3:1) establishes that two tzara’at infections are not dealt with simultaneously. Therefore, if one of the two connected spots has two white hairs, the individual is considered a metzora and completes the entire process required of a person stricken with tzara’at, and only thereafter is the second spot (assuming it is still present) examined. And if the second spot qualifies as a definitive tzara’at infection, then the individual must repeat the entire process. If, however, the line connecting the two spots is the width of two hairs, then the two spots are treated as a single discoloration, and the presence of two hairs in one spot – or, for that the matter, the spreading of one spot, which also confirms the status of tzara’at – renders the pair of spots a definitive tzara’at infection, and the individual must undergo the process just once.
The Kessef Mishneh commentary to the Rambam (Hilkhot Tum’at Tzara’at 4:7) adds that this question comes into play also in a different case – namely, if a single white hair appears in one discoloration, and another white hair appears in the other. If the white strip connecting the two spots is too thin, such that they are considered separate discolorations, then neither is definitively confirmed as a tzara’at infection in this case, as each contains only a single white hair. If, however, the strip is wide enough to halakhically combine the two areas, then we would regard this situation as one of a discoloration containing two white hairs, such that the individual would be declared a metzora.