SCAB (יַלֶּ֫פֶת, H3539, מִסְפַּ֫חַת, H5030; LXX Gr. σημασία; λείχην). A scab is a crust on a sore. Of itself it is not alarming, and in certain cases may even have a protective function. The important thing is the type of sore beneath the crust.
In OT days a person with a persistent scab was obliged to report to a priest, who would determine whether it was a non-progressive, temporary, benign condition, or something that required isolation (Lev 13:2-8).
Priests were disqualified for service, and sacrificial animals were rejected, if a scab was present. “Scabby mouth,” with sores around the lips, nose and eyelids of sheep is a condition known to veterinarians today and may well have been a familiar sight in the hot, dry climate of Israel.
The daughters of Zion were to be punished with scabs on the head (Isa 3:17) if the people failed to worship the Lord. We only have to think of a child with generalized, greasy, crusty cradle-cap (seborrheic dermatitis) to realize what a distressing, humiliating affliction this would be for a young woman.
International Standard Bible Encyclopedia (1915)
skab, skab’-ed, skabd (yallepheth, micpachath, cappachath, verb sippach; semasia, leichen): These are generic terms for any skin disease in which there are patches of hard crusts on the surface. The commonest of these are the forms now named eczema, herpes and, perhaps, psoriasis, all of which are common in Bible lands. Milder cases in which the disease was localized and in small patches (the semasia of the Septuagint) did not render the bearer unclean, and they were to be distinguished by the priest (Le 13:2,6) from the more virulent and spreading eruptions which (Le 13:7) were regarded as causes of ceremonial uncleanness. These severer forms are the leichen of Septuagint mentioned in Le 21:20, which disqualified any son of Aaron from serving as a priest, and when affecting an animal rendered it unfit to be offered as a burnt offering (Le 22:22). Hippocrates speaks of these cases as obstinate and persistent, and Galen believed that they might degenerate into leprosy; hence, the terms in which Aeschylus speaks of it (Choephori 281). Celsus, however, recognized that leichen was a papular eruption, not a true scab. The name yallepheth seems to have been given to it on account of the firmness of attachment of the scabs, while the term micpachath refers to its tendency to spread and cover the surface. A cognate word in Eze 13:18 is the name of a large Tallith or prayer veil used by the false prophetesses in Israel (translated "kerchief"). Scabs were especially disfiguring on the head, and this infliction was threatened as a punishment on the daughters of Zion for their wanton haughtiness (Isa 3:17). In Middle English, "scab" is used for itch or mange, and as a term of opprobrium, as in Greene, Bacon and Bungay, 35, 1591.