The most noteworthy point observed in our studies is the extraordinary duration of life and the relatively meagre requirements for sustenance of Leptospira icteroides. It is conceivable that under natural conditions opportunities might arise for the prolonged existence of Leptospira icteroides, so that possibly after a lapse of years, the disease might reappear without introduction from outside. At any rate, the great viability of Leptospira icteroides must be considered in this connection.
A decrease in the virulence of the leptospira does not rule out the possibility of a renewed outburst of yellow fever, since Uhlenhuth and Zuelzer have shown that it is possible to set up Weil's disease in guinea pigs by means of apparently saprophytic spirochetes obtained from drinking water, when the virulence of these organisms has been artificially heightened.
