I don't know about the Japanese, but in Europe where this practice was common (certainly in London) one answer is simply that they tolerated more outbreaks of i

I don't know about the Japanese, but in Europe where this practice was common (certainly in London) one answer is simply that they tolerated more outbreaks of infectious disease. It's possible to make it safe through composting, but that requires more time and skill than a typical compost heap. Most composting authorities recommend keeping cat and dog faeces - less dangerous than human waste, but much worse than cow manure - out of the garden compost heap.