1.The virus of yellow fever occurs in the blood of experimentally infected rhesus monkeys in very high concentration: of eighteen specimens of blood taken from infected animals at the onset of fever and tested in dilution of 1:1,000,000, all proved infective; and of six specimens tested in dilution of 1:1,000,000,000, three produced fatal infection in normal monkeys.
2.A considerable variation in susceptibility was observed in these monkeys when a highly virulent strain of yellow fever virus was injected in very high dilutions, and very small amounts of the virus were frequently found to immunize animals without producing any outward signs of infection.
3.In the majority of the animals inoculated with minute quantities of virus the infection was characterized by a rather long incubation period, followed usually be a sharp, short febrile attack and death.
4.Human hydrocele fluid from persons not immune to yellow fever, either concentrated or diluted 1:1 with sterile physiological saline, was found to be a satisfactory diluent for the virus for titration purposes.
5.The development of yellow fever antibodies in the blood of experimentally infected animals was found to take place early during the course of disease. The serum taken at death from monkeys that died on the fourth, fifth, and sixth days after inoculation showed no protective properties, but the serum of animals that died on the seventh day or later protected fully against massive doses of virus.
6.The serum of monkeys that were inoculated with very minute doses of virus and died of yellow fever after a long incubation period, showed no protective properties.
7.The virus was found to possess a relatively high resistance to the action of post-mortem invading bacteria. The blood of a monkey that died of yellow fever and was kept after death for nine hours at a temperature varying between 78 and 86.5° F. was still found to be infective in 0.01 cc. amounts, although the organs of the animal at that time were already in an advanced state of decomposition.