In short, not great. Of the approximately 140,000 (white) PoWs that were prisoners in the camps, around 1 in 3 would die of starvation/disease or over exhaustion from being worked, quite literally, to death. Those who survived the Bataan death march for example, after the c. 6,000 - 18,000 deaths on the march had arrived in Balanga, many then subsequently died of dysentery. PoWs were not provided with any medical care or supplies so a number of US personnel attempted medical care with the little supplies they had.

Many camps were overcrowded, which added to the ease of the spread of diseases between prisoners. Most camps were in the form of barracks and had barbed wire surrounding them - of the few escape attempts, once caught they often executed other prisoners as well as the escapees. They often made their own ‘rules’ and punished the prisoners according to these, with many being subject to torture on numerous occasions. Most guards couldn’t speak English and prisoners were forced to learn Japanese to understand the commands given to them to avoid being beaten.

As most were put to work, some former prisoners who managed to survived commented that they were “white slaves, on a diet of 600 calories a day. Working 12 hours a day on a diet of soya beans and seaweed” (Henry Carver, a former prisoner). Arguably the ones who suffered the most under these work conditions were those sent to build the Burma-Thailand railway, expected to build the 260 mile railway by hand and on a diet of less than 600 calories while working 10 days straight, from dawn until dusk with little rest.

The most shocking of conditions were arguably found in their Unit 731, a “prisoner of war” camp that housed mostly citizens that had been rounded up by the military police. The unit was their biological warfare research centre and the prisoners there were subject to horrific acts under the guise of “research”: people were infected with STDs such as syphilis and then cut open whilst alive and awake to study the effects of the disease on the human body at different stages. Some people were also forced out into the cold, their limbs frozen and then beaten with hammers to see the effects of frostbite. Children were born with diseases and defects as a result of the researchers interest in the effect of diseases on pregnant women and their unborn children. Prisoners were also raped, deprived of food and injected with animal blood, amongst other horrific things.

The conditions in unit 731, and other camps were so horrific that many Japanese PoWs feared reprisals on them by Allied troops upon their capture.

An interesting couple of things to read about this subject further are:

Sheldon Harris ‘Factories of Death’ (2002)

Gavin Daws ‘Prisoners of the Japanese: POWs of WW2 in the pacific’ (1996)

Are two of my recommendations but there are a number of first hand accounts made into books and there are also lots of journal articles I can link you too as well!