If you are in a body of water that inflicts disease and leap/glide out of it, you will continue to gain disease until you land on solid land for a second or two. You still gain disease if you are above solide ground, but not touching anything (via gliding).
I can corroborate this bug as well to an extent. The game seems to think that you're in contact with the swamp water still if you jump or are near the surface after gaining Blight progress.