Infested Leaves is a block added by Ex Nihilo.
Infested Leaves can be created by right-clicking on Leaves with a Silkworm. Infested Leaves will slowly become white and can convert neighboring Leaves to Infested Leaves (with time, the "infestation" can spread to the entire tree and to neighboring trees).
Infested Leaves can be mined to drop String (and Saplings with the original probability). Infested Leaves will drop String, which probability being directly proportional to how white the block has become.
Mining Infested Leaves with a Crook significantly increases drop rate.