_Maculo-anæsthetic leprosy is therefore characterized by an infection of the skin and nerves directly caused by the lepra bacillus, and by secondary tropho-neurotic affections of the muscles, bones, and joints, the skin and organa sensus._ As is clear from the description of the two forms, the leprosy bacillus is found in the leprous products in both, though in much greater quantity in the nodular than in the maculo-anæsthetic form. The course of the two differs; in that in the nodular form eruptions constantly recur, and thus the affected areas are much more numerous than in the maculo-anæsthetic. Whether the liver, spleen, and testicle are attacked in the latter form we cannot as yet say with certainty. In one case of maculo-anæsthetic leprosy, we noted an affection of the throat exactly corresponding to that constantly present in the nodular form, and in another we found the inguinal glands affected with leprosy in a manner similar to that in the nodular form. In the maculo-anæsthetic form eruptions are also present, but they are by no means so frequent or so severe as in the other. Both forms may recover. As we have frequently noted in the description of nodular leprosy, the bacilli in the leprous products break up into granules which finally disappear, and there remains of the leprous products only a scar in which nothing leprous can be recognised. Occasionally this takes place in all the affected parts, and there remains only a widespread anæsthesia, the result of the nerve affections; and in the maculo-anæsthetic form this is the regular termination of the disease. In both cases the leprosy is completely healed. We are thus enabled to see how complete is the parallel between the two forms. The course of the disease, especially the eruption, gives to it evidently the character of an _infectious disease_. In the nodular form, where the bacilli are present in millions or milliards, the eruptions or auto-infections are frequent; in the maculo-anæsthetic form, where their number is comparatively insignificant the eruptions are rare. _Does this difference between the two forms depend on a difference in the virulence of the bacilli?_ This readily suggests itself. But if so, this virulence is capable of very rapid changes. We have seen a case of maculo-anæsthetic leprosy, which probably arose by inoculation from a very severe case of nodular leprosy, since the patient some years before the outbreak of the disease had for a long time shared the bed of a nodular leper. In this case the virulence of the bacilli must have been at once diminished on their inoculation on another organism. And since it also happens that a maculo-anæsthetic case may on a fresh eruption become nodular, the bacilli must be able by cultivation in the organism to re-acquire their power. Both are possible, but the virulence of the bacilli seems to depend, not so much on any constant character of their own, as on the soil in which they live.