The ratio of sickness to battle casualties during the early campaigns in New Guinea was approx. 16-30 to 1. In the battle over the Owen Stanley Mt. Ranges on the Kokoda Trail an epidemic of dysentery broke out. All the available sulfa-guanidine was flown from Australia to Moresby and thence to the Regimental Aid Posts on the trail, and sulfaguanidine, in 4-gm. doses, was given to each soldier with diarrhea or dysenteric symptoms. Within 10 days the epidemic was under control. Apparently, this drug controlled the diarrhea and inhibited the multiplication of Shigella dysenteriae, so that finally the mortality rate was reduced to 1:5000. In the thousands of cases treated, no single instance of agranulocytosis, anuria, or dermatitis occurred. During this same period many thousands of Japanese soldiers were dying from dysentery in the battle area. In the Southwest Pacific area, the av. mortality rate from scrub typhus was 8% with a mortality rate in some areas of 30%. There was no specific therapy for scrub typhus except preventive. Dibutyl phthalate was sprayed on all clothing and blankets. In New Guinea this compound was found superior to dimethyl phthalate because its larvicidal effect persisted longer and provided protection against the scrub itch as well as the mite typhus. Scrub typhus was transmitted by the bite of larval trombiculid mites containing Rickettsia tsutsugamushi. Dengue fever, although not fatal, occasionally prostrated as much as 60% of a military force. In New Guinea, Aedes scutellaris was found to be a new vector. The number of cases of dengue was directly proportional to the density of this mosquito around the camp site. Exptl. transmission to man was studied at the Univ. of Sydney from mosquitoes obtained in the Finschafen area of New Guinea. Three vqlunteers received 32-82 bites from the A. scutellaris and each developed dengue; other volunteers, bitten by Armigeres breinli, milnensis, and Aedes auremargo, also obtained from New Guinea, failed to develop dengue. There was no specific drug. Control depended on prevention by protective clothing, repellents, and mosquito control. Malaria casualties were extremely heavy in Milne Bay, Buna Gona, Ramu, and Markham Valley. A medical Research Unit was formed to investigate the malaria problem in New Guinea. Volunteers who received 10 grains of quinine daily were exposed to the bites of mosquitoes containing the sporozoites of Plasmodium falciparum; all developed malignant tertian malaria. In volunteers exposed to the anophelines with P. vivax, 10 grains of quinine provided satisfactory suppression in 2/3 of the cases. Volunteers exposed to mosquitoes infected with P. vivax did not develop malaria while taking atabrine; however, within 14-44 day after the drug was stopped, clinical malaria appeared. In malignant tertian malaria, atabrine acted not only as a prophylactic but as a curative agent. In these expts., parasites were never demonstrable in the blood. Further, subinoculation with 300 ml. of blood in fresh volunteers also failed to produce the disease. Attempts were also made to produce the mixed type of malarial infec- tion. Soldiers were exposed to mosquitoes harboring P. vivax or P. falciparum, as well as to physical fatigue, cold, anoxia, and loss of blood; in no case did malaria develop while atabrine was taken, but 30 days after the drug was stopped, benign tertian malaria appeared in every soldier. In no case were the parasites of P. falciparum found. These expts. showed that troops could fight in hyperendemic areas of malaria for indefinite periods, provided that atabrine was taken daily.