A rot grub is a 1 inch long grub that eats rotting organic material. They are often found in the bark of treants in the forest of Cormanthor where they help to combat the tube wilt by eating the rotten wood and the fungal infection that causes it.
Outside of Cormanthor, they are most often found in piles of offal or dung, attracted by movement, they will burrow into the nearest moving object and eat whatever they can. If the victim is quick, the problem can be treated with fire or by cutting them out with a blade. If not, a remove disease spell will get rid of them before they reach the victim's heart.