Assassin's Creed III Remastered

The Witch Doctor will give you a side quest that will open these missions up. The mission is called "De Bayou Fever". You will need to collect mushroom in De Bayou and give them to people inflicted with the fever. The people who have the fever show on the map as skulls and when you see them they are flailing themselves in a crazy manner. The good news is you will be collecting the mushrooms for the collector achievement anyway. See the "Collector" achievement for a map of their locations.