August 31st, 2021 at 2:12 AM
From what I've noticed in the game, sickness seems to come at random. It might be possible to stop sickness from ever spreading, because that seems to have happened in my largest town from my most recent time playing banished in April. Single people got sick twice and both times they immediately went to one of the physicians I had built and recovered before anyone else became sick. It might have also been luck of course.