Those who would like to interpret the ritual purity system in medical terms find their best evidence in the impurity of leprosy. Here is an impurity which is definitely a disease. Of the other forms of impurity only the zab and the zabah are in an abnormal state of health. But their condition is not usually regarded as infectious, so procedures minimising contact with them do not seem to have a hygienic purpose. In the case of lepers, however, many societies have isolated them in the belief, however exaggerated, that they are dangerously infectious. Some have seen corpse-impurity as a hygienic measure. Especially in hot climates and places where infectious diseases are rife, it has been argued, there is danger in contact with a corpse. But contact with a corpse is not forbidden except to the priests. Others are actually encouraged to take part in the necessary work of preparing the corpse for burial. The impurity of the corpse is evidently part of the protection of the Temple from impurities, not the protection of people's health.

But how serious and how infectious is leprosy? It must be said at once that the skin affections described in Lev. 13 are not leprosy in the modern sense of the term (Hansen's disease). Indeed, the symptoms described belong not to one, but to many, skin affections, all of which were lumped together under the Hebrew term tzara'at psoriasis, favus, vitiligo and others. The only reason why the term ‘leprosy’ was used to describe these ailments is a confusion (probably not perpetrated until the ninth century CE by John of Damascus) about the Greek word lepra, which is used both in Septuagint and NT.