Cockroach

The Sims
Cockroaches were introduced in Livin' Large. They infest houses when it is dirty, and once a swarm appears, they can be hard to get rid of. Spraying them can temporarily kill them, but as long as there are still dirty items in the house, they will keep coming back.

The Sims 2
In The Sims 2, cockroaches, more often referred to as simply 'vermin,' will infest an extremely dirty area (usually trash left outside for too long). They can be killed by spraying them or calling an exterminator to kill them, but after that the trash (or whatever else attracted them) must be cleaned up or they will return continuously. Sims will get bad memories from seeing vermin and will often cry about having seen them even after the vermin are gone.