Plague is an offensive spell used by the Zerg Defiler that affects any unit or building with a 4x4 matrix of the target. A unit affected by a Plague slowly begins to lose hit points (at about 11.8 damage per second in Fastest game speed) until either the maximum of 300 points of damage is done or the unit reaches one health. Plague remains in effect for 25 seconds in Fastest game speed. Plague alone cannot kill anything, but the effects can still be rather crippling. Plague can allow you to kill powerful units with one hit from another unit, an example being a Mutalisk finishing off Science Vessels with one hit. Plague is particularly devastating to Terran Buildings, as it will continue to take further damage from burning if the building has less than 1/3 of its original hit points. The Protoss are able to better withstand Plague, as their Psi-based Shields are unaffected by the biotoxins of the Plague. Plague does not affect burrowed units, but if a unit is infected, then burrows, the unit will still continue to take damage. The viscous biotoxins of a Plague will also stick to Cloaked units, revealing them as long as the Plague lasts. 
In competitive play Plague is a costly but devastating spell. Its gas-heavy and high-tech requirements make Plague unavailable until late game in which it often plays a great role if gas resources allow it. Because of the cost and tech it generally isn't found in ZvZ.
Plague is often deployed against MnM squads, the units in which are frequently clumped in great numbers. Effectively Plaguing the bulk of them often sends them scampering in retreat as they stand no chance versus the Zerg onslaught. Additionally, Medics will in vain deplete their energy, making them useless for a good while after. When the Terran gets a fair number of Vessels these clouds become the target of plagues. At this point in the game the Zerg often gets a single Mutalisk to snipe the near-dead Vessels.
Plague generally isn't used versus Terran Mech builds. The game usually remains at Lair tech, the bigger units make the Plague's Area of Effect less deadly and Defilers are easily sniped.
Though not as effective against Protoss (who still have their shields), Plague can still be used in this match-up to attack the Protoss armies and bases. It isn't commonly used against Dragoon-heavy armies as Dark Swarm would be the preferred spell. The efficiency of plague in late game vs Protoss is that it can twice decrease vitality of their units without decreasing their Psi.
Despite the high cost and commonly unused tech branch, Plague can be used in late ZvZ. It can be used in conjunction with adrenalings (aka Cracklings, Zerglings with Adrenal Gland upgrade) under Dark Swarm. The damaged units will have little health, and the fast attacking Zerglings will clean up the enemies easily. Consume is a useful way to get the energy of the Defiler to 150, the energy cost of Plague. Plague can be applied to large groups of enemy air units, which are common late ZvZ. If you opt for plague tech, it becomes important because the enemy air army will be larger and stronger than yours.
In Combination with Other Spells
- Stasised units cannot be Plagued. Plagued units that are stasised will stop receiving damage and Plague will wear off after the remainder of the 25 seconds.
- Plague can be "recalled" by enemy's Arbiter. This Plague + Recall replay is an example of Plague + Recall effect that was caused at about 39:10 when Recall from center to 3 o'clock natural was done and Plague was casted to center. The animation of Plague was shown at center but effect (units and building affected) was applied at 3 o'clock natural. (recalling Arbiter was affected too).
- Plague will dispel Hallucinations.
- Plague damage will stack with damage from Psionic Storm and Irradiate, but not from another Plague.
- Plague will damage units in Maelstrom or Lockdown.
- Plague can be cured with Restoration.
- Defensive Matrix will not prevent or influence damage dealt by Plague
- Defilers can't Plague themselves.
- Plague will carry over to both types of Archon if one of the Templars was Plagued.
- Plague will carry over to Zerg buildings and morphing units.
- Plague will damage units in a gas station or a transporter.
- Protoss shields are unaffected by Plague.
- Interceptors flying back to a Carrier will heal up and be sent out when they are back at full health. Casting Plague on the Interceptors will cause them to remain in the Carrier until the spell wears off, consequently disabling the Carriers from attacking. This "bug" is somewhat debated but perfectly legal in Professional StarCraft as well as on ICCup.
- https://www.youtube.com/watch?v=Vpuv7VPb2rA - PLAAAGUUUUUUUEEEEE
- adapted from: Zerg Strategy Guide <http://classic.battle.net/scc/zerg/special.shtml>