Japanese and Australian medical problems
It has been said that epidemics carry the blame for defeat while the generals get the credit for victory...
It may be that the initial defeats suffered by the Australians in Papua can be blamed on sickness, just as the final defeat of the Nankai Shitai (the Japanese force in Papua) in January 1943 had its roots in the same cause.
Prior to this, in the mountains along the Kokoda track and at Milne Bay, it was the Australians who suffered more from illness than the Japanese. It was not until November of 1942 that Japanese medical problems became worse than those of the Australians.
The Japanese came to Papua better prepared to combat disease than the Australians. An Australian study of 1943 stated that the Japanese were the 'most inoculated army in the world.' To the end of September the Australians had evacuated 1752 sick men from the mountains against 343 Japanese. The turning point was November and, by December, Japanese casualties to sickness had passed the Australian numbers. The reason for this can be traced to Japanese problems with supply. Most of the Nankai Shitai were long-service veterans who had had malaria several times before in China, or more recently in hyperendemic Rabaul, where the Nankai Shitai had spent the six months prior to coming to Papua. Provided quinine was regularly supplied to the troops malaria could be kept at bay. However when the supply line was interrupted a malaria outbreak occurred. By December, a Japanese report stated, just one third of the Nankai Shitai were fit and in January 1943, at the end of the Papuan campaign, the portion of fit men in the force dropped to one seventh of the total.
The health of all armies begins to decline as soon as they leave their barracks. In healthy environments this decline is slow but New Guinea was not then - and is not now - a healthy environment. As soon as soldiers take to the field, even if they are not in battle, their health will begin to decline. Japanese, American and Australian studies at the time all concluded that three months in New Guinea brought about a noticeable decrease in the effectiveness of a unit regardless of whether or not it had been in battle. After six months exposure to the health hazards of the environment any unit would be of little fighting value.
For the Australians on the Kokoda track the main problem was dysentery. An outbreak began in mid-August near Ioribaiwa and spread up and down the track from there, arriving at the front line at the end of the Battle of Isurava as the Australian retreat began. For the next two months from 30 to 80 men each day, from a force varying in size from 2000 to 3000 men, were evacuated sick, mostly with dysentery. In November the Australians advanced out of the Owen Stanley Range, where there was no malaria, down on to the lowlands about Gona, Buna and Sanananda. The dysentery epidemic had been brought under control just as malaria began to take hold. By 27 December 4857 Australians had been evacuated sick from the front line. This was more than half of all the Australians who had fought in the campaign to that point.