Guild Wars 2

Say my teammates inflict an enemy with conditions and I use Epidemic to spread those conditions to other enemies, would I get healed for these conditions? Or does it only apply to the ones I inflicted myself?

Indeed you will.
When you epidemic someone the conditions spread become “yours” and based on your stats and traits.

The more people have conditions inflicted upon them by you, the more healing you will get in return. In WvW for example, if there is a zerg clash, hopping in to Plague form and tagging as many people as absolutely possible (assuming you have lengthy condition duration) will give you a lot of return. I’m able to make my health go up faster than it goes down even when I have every condition in the game piled on top of me. It’s a great trait for large scale battles assuming you have good condition duration, but in small scale combat it rarely shines.

Epidemic does not actually spread conditions literally. It acts just like a condition transfer, from the targeted enemy to up to 5 nearby enemies (but without the cleansing), and just like with condition transfer the mechanics work out such that you apply the condition, just without it being modified by duration increase.