In The Sims, flies appear over plates of spoiled food, and eventually appear over piles of trash, dead mice[TS:U], and pet droppings[TS:U]. In addition, "free-range" flies often appear if the house or lot is messy—even unmade beds can cause them to appear. These flies are not harmful, but when they are in a room, they reduce their Room score.
If Unleashed is installed, dogs will sometimes jump and snap at "free-range" flies, but cannot get rid of them. "free-range" flies can be picked up and sold in Buy or Build mode if the move_objects on cheat is enabled. Oddly, they're labeled as the "SSRI Virtual Reality Set" and worth 460 simoleons.
The Sims 2
In The Sims 2, flies will appear over plates of spoiled food, and will eventually appear over piles of trash, very dirty toilets, Sims who have been at 0 hygiene for 1–3 days, and dirty pet bedding[TS2:P]. Babies and toddlers that have a sloppy personality will also have flies appear alongside green stink clouds by their butt whenever they have a dirty diaper. It does not seem as though "free-range" flies exist in The Sims 2. However, having too many fly-bearing objects on a lot can result in "death by flies".
Death by flies
Being eaten alive by flies is a form of death in The Sims 2. The event can be triggered if there are many (10-15) dirty, fly-infested objects emitting green whiff clouds in a room. If a Sim walks through this pest breeding ground, there is a chance that the flies will swarm them and eat them alive until nothing of the Sim's body remains. This event is more likely to happen to Sims with low hygiene. How flies acquired the ability—or the incentive—to do this is not explained. The eaten Sim's ghost will be purple.
The Sims 3
In The Sims 3, flies hover over food or dishes that have not been put away or washed in a few Sim days. However, flies do not kill Sims in this game, but can cause Dirty/Filthy/Vile Surroundings moodlet and can be a major annoyance to the player.
A glitch in The Sims 3 for Nintendo DS causes flies to be permanently stuck in the house, even if all the dirty dishes and garbage have been removed.
The Sims 4
Flies in The Sims 4 are similar to the ones in The Sims 3. They hover over spoiled food and empty plates that haven't been put away or cleaned. As with The Sims 3, flies do not kill Sims in this game's base version, but they can make them uncomfortable. Flies will also appear around foul-scented laundry in The Sims 4: Laundry Day Stuff.
Death by flies
Being eaten alive by flies is a form of death in The Sims 4: Eco Lifestyle. Unlike The Sims 2 equivalent, the Sim's body stays intact upon death. This death can occur if the Sim tries to do the Bond with Flies interaction. The ghost appears to always be surrounded by flies.
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