Fire is in need of cleanup for the following reason(s):
Better writing needed
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Fire is an effect that only occurs in Terran structures. Fire is a debuff that causes a structure to consistently take a small about of DPS, until the building destroys itself or is repaired. Fire always occurs when the structure has about ~1/4 of it's health remaining, and occurs sometimes when ~1/3 of it's health is remaining.
When playing as Terran, remember that large amounts of fires can really ruin your day if you don't prioritize repairs for these structures first. The same goes for your opponent. Gain the upper hand by causing structure fires on enemy buildings. This will allow weaker units to finish them off when you go for the clean-up sweep. If your opponents decide to use Lift Off to avoid incoming attacks, attack them to the point of causing a fire. The structures are slow moving, and will probably destroy themselves before they can find a landing spot. However, be aware that the same can happen to your structures, and that they might have some SCVs handy if it ignites.
In the campaign, Fire-suppression drones are available for purchase at the Armory for a hefty price of 90,000 Credits. The drones will put out fires and repair buildings and building Add-ons to half health. An SCV is required to repair any structures past this point.
Unlike real fires, fires in StarCraft I and II do not spread building to building. The damage debuff does not stack as well. Fires cannot be caused by units using fire-based weapons (i.e: Your units cannot cause a fire if the building is above 1/4 health, regardless if they use fire weapons).