Leech Blood is a resource in Ark Survival Evolved that is used for both fishing and the creation of antidotes which can cure diseases. Leeches are found in the swamps and that is also where you can contract the diseases which require the antidote.
To get Leech Blood, the easiest way is by harvesting Leeches, which are found in swamp regions. You can also get small amounts of Leech Blood as a rare drop when fishing. Care must be taken when harvesting Leeches in the Swamp as you can run into Diseased Leeches, which will give you Swamp Fever. Then you will need to make an Antidote.
Getting Leech Blood in Scorched Earth is a much more difficult matter. On the Scorched Earth map your only sources of Leech Blood are rare fishing drops and harvesting a Deathworm. These creatures are extremely dangerous, but if someone has brought disease to your Scorched Earth map, this is your only real chance to get the Leech Blood you need.