=Etiology.=—The spirochaete causing infectious jaundice, _Leptospira icterohaemorrhagiae_, is the type species of a genus described by Noguchi as having minute elementary spirals running throughout the body and failing to show either flagella or undulating membrane. The caudal portion of the spirochaete is remarkably flexible and when in motion the whole body seems drawn into a straight line except for the hook formation of one or both terminal portions. Propulsion seems to occur by the rotary motion of the hook and progresses in the direction of the straight end. If both ends become curved progression ceases. [Illustration: FIG. 49.—Four specimens of _Leptospira icterohaemorrhagiae_. (After Noguchi in Journal of Experimental Medicine.)] [Illustration: FIG. 50.—A leptospira viewed under the dark-field microscope. (After Noguchi in Journal of Experimental Medicine.)] It is insoluble in 10% saponin thus differing from the other blood spirochaetes. The constituent spirals are closely placed and the total length may reach 15 to 20 microns. The dark-field illumination is preferable for its demonstration although it shows up well by various staining methods. To culture use a medium of one part of rabbit’s serum with three parts of Ringer’s solution inoculating with citrated plasma. The organism is found in the blood during the first three or four days of the disease. It is also present in the urine. Young guinea pigs are particularly susceptible and in them we have, following injection of the blood of a case, jaundice, albuminuria and haemorrhages. At autopsy the spirochaetes are best demonstrated in a liver emulsion. Infection with this spirochaete is common among rats in various parts of the world so that it may be considered a natural infection of rats, but they do not seem to suffer from it. =Epidemiology.=—It is considered that the infection is transmitted through the urine of infected rats or men. The spirochaetes may gain entrance through the skin or by mouth. The fact that the spirochaetes seem to die in urine within 24 hours makes infection by the contaminated urine questionable but experiments with various arthropods have failed to indicate their connection with the transmission of the disease. In Japan it has been noted that the disease is most common in wet mines, disappearing when the mines are pumped dry. Before we knew the cause of the disease attention was directed to the connection between the infection and working about defective sewers. Trench warfare, with the accompanying rats and wet conditions, seemed to favor infection. PATHOLOGY The liver shows enlargement and fatty degeneration, but less marked than in yellow fever. The organisms are found in the liver and also in the kidneys, which latter show an intertubular congestion together with haemorrhages in this region. Haemorrhagic areas may be present in various portions of the alimentary tract and elsewhere. SYMPTOMATOLOGY