Infestation is a Scrin support power, enabled by construction of the Nerve Center, that allows them to deploy a Tiberium hive at a selected Tiberium field, damaging any unit, friendly or hostile, to traverse the affected field. The Tiberium growth at the field is also slowed.
Infestation is primarily used to slow down an enemy's economy, as it forces them to relocate their Harvesters to a new Tiberium field, which may leave them to open attack if said field was outside the enemy's base. It may even destroy these Harvesters if the enemy is either unaware(due to battle) or ignorant of his economy going down.
Infestation may also be used to force the enemy to make two choices when an infested Tiberium field is blocking the path to the player's base: either go around it, which buys the player time to prepare defenses, or go through the field and risk losing their units to destroy the Tiberium hive. AI enemies are known to use the second option, often either ignoring or destroying the Tiberium hive just to reach their targets. Also, in maps such as Bordertown Beatdown and Rocktagon, this is very effective, though at the cost of turning a potential economy boost into a deathtrap for all sides.
Because the Tiberium hive is almost as fragile as a Growth Accelerator, Infestation is not a long-term solution, as a daring commander can send his units durable or fast enough to destroy the hive. Air units and some support powers such as Orca Strike can destroy it without risk.
Note that Infestation is only effective on Tiberium fields that are not depleted, because otherwise it would be nearly useless until the Tiberium grows back to reasonable amounts.