These can be found in the ponds to either the north or south of the crypts.
Travel to the Bubonic Boneyard near ( 262, 65, -344 ) /waypoint 262, 65, -344 and release the clot water leeches near bubonians. Collect 6 cyst-covered bubonian skins after they die.
Get about 10-15m away and "use" the leech out of your inventory.
It's best to approach the bubonians from behind so they don't see you and aggro.
The mob has a slightly delayed reaction, then will become non-aggro and after few seconds of writhing and vomiting will fall over, at which point you can click on the body.
If the mob wanders away before eating the leech, you can right-click the leech and pick it back up.
If the mob aggros while you're dropping the leech, and you run it off, it will trigger on the leech when it returns and then die. BUT it will probably be bugged and you won't be able to get an update. And your leech will be gone. Best to kill it if it aggros.
Speak with Kanlum at ( 497, 83, 47 ) /waypoint 497, 83, 47.