During the monsoons, it rained heavily and continuously. The ditches, pits and channels all overflowed with water...
In the (....) I want to say something like:
this whole scene of torrential rainfall promoted unhealthiness, due to which the postmaster's health deteriorated.
Although it doesn't sound quite right, is there a better way to put this in?
EDIT: The whole story takes place in a village. The protagonist, who spent all his life in a city, comes here for his first job. Although due to his background he's unable to mingle with the other residents of the area. As a result, he feels lonely and out of place. Due to the abundant rain of the monsoons, the whole place develops a very gloomy ambiance – which promotes illness (I hope you get the idea).