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