The lightcrawlers in disease don't actually heal themselves, it's the NPC regen rate triggering. This seems common with many planar mobs - have it happen to me when chant-kiting anywhere in the Planes, infact.
Whenever the mob gets to 'running' mode, if you don't stay within a certain (relatively close) distance, it will kick in the NPC uber regen until it reaches 20 to 30% health. I just stay as close as possible without getting hit once it's down to about 15% or so, then pay close attention to when it starts running, so I don't run out of range.
Another way to do it would be to purposely get an add when your first one is low, so their social code kicks in and they never start running.