From time to time diseases appear in your settlement. Some diseases occur as a consequence of lack of facilities in your settlement. Some are just seasonal diseases that can't be affected much. When your ship goes for explorations, her crew can bring more severe versions of seasonal diseases back to the island.
Each farm can be set up with a crop rotation schedule up to 4 seasons long. Crops will be grown in order from left to right, any missing crops will be automatically skipped. For example, to set up a two crop rotation select two crops in the schedule and leave everything else blank. Crop rotation can be important for farm’s fertility.