In South America, the disease is found in Columbia, Venezuela and Brazil as is also true of Mexico and Central America. In Australia the disease is found in Queensland and New South Wales. It also prevails in New Caledonia and the islands of the Pacific. ETIOLOGY AND EPIDEMIOLOGY =Etiology.=—Leprosy is caused by an acid-fast bacillus, _Bacillus leprae_, which rather closely resembles the tubercle bacillus morphologically as well as tinctorially. It was first discovered by Hansen in 1871 and fully reported in 1874. Much of our knowledge of its characteristics is due to Neisser (1879). The leprosy bacilli are found in profusion in the granulomatous tissue of the corium and subcutaneous structures of the leprous nodules, chiefly within cells called “lepra cells” and also within endothelial and connective-tissue cells as well as lying free, packed in lymphatic channels, the so-called “globi.” The leprosy bacillus may be distinguished from the tubercle bacillus by the following points: 1. The presence ordinarily of huge numbers of bacilli often grouped in packets like a bundle of cigars tied together. It will be remembered that it is very difficult to find even a single tubercle bacillus in a skin lesion. Leprosy bacilli form palisade groups but not chains. 2. The leprosy bacilli stain more solidly and when granules are present they are coarser and more widely separated than the fine granulations of the tubercle bacillus. 3. They do not stand decolorization quite as well as the tubercle bacillus. With 20% sulphuric acid in water they hold their color almost as well as tubercle bacilli but with 3% HCl in alcohol they decolorize in about two hours as against twelve to twenty-four hours for the tubercle bacillus. 4. Leprosy bacilli have neither been surely cultivated nor surely inoculated with pathogenic results into guinea pigs or other experimental animals and it is by the negative results upon cultivating or animal inoculation that we have our surest method of differentiation from tubercle bacilli. Leprosy bacilli are chiefly spread through the lymphatics, but in nodular leprosy, their occurrence in the blood stream during the febrile accessions is so constant that this route may also be of importance. Next to the corium they are most abundant in the lymphatic glands. They stain readily by Gram’s method. A great amount of work has been done within recent years in attempting to cultivate the leprosy bacillus.