_The Blood Examination._—This is the most important point in diagnosis. The spirochaetes, which are only found in the peripheral circulation during fever periods, are not so numerous in tropical relapsing fevers as in the European forms. When spirochaetes are scarce it is more satisfactory to examine Romanowsky-stained specimens, especially with the Giemsa staining. The spirochaetes show a varying number of undulations. There is no chromatin staining in the line of the spirochaetes. The disease when severe shows a well-marked polymorphonuclear leucocytosis, with at times an increase of large mononuclears. This latter, however, may be connected with malaria or amoebiasis. DIAGNOSIS The disease most likely to be confused with relapsing fever is malaria and for this differentiation the finding of the parasites of either disease is of first importance. The blood of a suspected case even during the apyrexial period should be injected into a mouse or white rat (guinea pigs are refractory to infection). Spirochaetes should appear in the blood of the mouse in about twenty-two hours and persist for about two days. Relapses occur but recovery is the rule. Dengue may be suspected, but the leucopenia, lack of splenic tenderness, lack of tendency to vomiting and presence of post-orbital pains should differentiate. As there is a leucocytosis in both relapsing fevers and smallpox, and similar headache and backache, confusion might exist were the parasites not found. Yellow fever has many features in common with the bilious type of relapsing fever, but there is no leucocytosis in yellow fever, and there is no characteristic albuminuria and slow pulse in relapsing fever. Influenza has many points in common with relapsing fever. In a case of relapsing fever with jaundice confusion might arise with Weil’s disease inasmuch as a blood smear might show spirochaete-like organisms. Typhus fever shows a less abrupt onset and the marked mental symptoms (stupor) and dark macular eruptions about the trunk, on the 4th to 6th day, should differentiate. If the case is first seen in the apyrexial period one may take a drop of blood from a case showing spirochaetes and one from the suspected patient. After incubation for thirty minutes the spirochaetes should lose motility and agglutinate if the case be one of relapsing fever (Lowenthal’s Reaction). In blood examinations we may use the dark-field illumination, although the spirochaetes stain readily with Wright’s stain. The India ink method is a good one. Hagler recommends smearing out a mixture of one loopful of blood and a collargol preparation made by diluting one part collargol with two parts water. The diluted collargol should stand 24 hours and be filtered before use. PROGNOSIS The mortality is usually given as about 2 to 5% with the exception of the very serious form in which jaundice is present when the death rate may exceed 50%.