One of the latest works from Greg Egan, the Australian master of strict SF. The events of this story take place in a world not quite similar to ours, where different settlements are out of sync with each other, and can only interact with each other when their natural cycles coincide. For example, eating food from another village, your body will accept it, but when the cycle ends, your body will begin to experience significant discomfort, and those healthy proteins that you absorbed will simply disappear. The various villages coexisted peacefully for a long time, but a new disease has appeared, which some people blame on the influence of other settlements.