Flies

Flies have been part of The Sims series since the beginning.

The Sims
In The Sims, they appear over plates of spoiled food, and eventually appear over piles of trash, dead mice, and pet droppings. 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 its Room score.

If Unleashed is installed, dogs will sometimes jump and snap at "free range" files, 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 and dirty pet bedding. 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. 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 has 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.