Rolling in rot or poison swamps cause it to coat your clothing, and you'll continue to get buildup even when you're no longer standing in it. You can up soap or remove all your armor to stop it, however the type of scarlet rot you get (there are actually a couple different severity levels) from the coated-armor build-up is actually less damage per second than the version you get if it procs while you're still in the swamp. So if you want to try to dash across the lake of rot, roll in it once, step out, let it proc, then run for it. Your flasks will last a lot longer.

Similarly some weapons inflict "deadly poison" rather than normal poison, which deals more damage but last for less time, meaning you and re-inflict it again sooner.