Leeches can be found in the Swamp Biome. When equipped to the Torso slot a Leech will periodically drain .2 Blood every 10 seconds. If you have Maroon Fever each time a Leech drains Blood there is a chance to cure the Disease. For advanced cases of Maroon Fever you may need to drain a significant amount of Blood. As a Leech consumes more Blood they will also get bigger. Right-clicking an engorged Leech and selecting "Drain" will give you .1 to . 5 liters of Blood of Pilgrim depending on how big it got. An empty Vessel, or one already containing Blood of Pilgrim must be in your Inventory when Draining a Leech, otherwise the Blood will be lost. A Leech can be stored in a Physician's Bag.
Once a Disease is Diagnosed, it can be treated with the appropriate medication. It may take more than one dose of the medication to cure the Disease, especially if the Disease has progressed particular far, or the wrong cure has been taken. CAUTION: Taking any cure will do .2 damage to the corresponding Humour. While small it is possible to become Knocked Out in this way.