“Infested” describes a problem with animals or insects in your home. It means that many of them are living in your home. Some animals that commonly infest people’s homes in North America include mice, slugs, and ants.

Annie saw a mouse on the second day, and we realized a few months later that the whole place was infested.

An infested home is a gross, unpleasant experience for anyone, and so “infest” has a strongly negative feeling to it. You can also sometimes use "infest" to describe a group of people you don’t like:

The bar was infested with Dodgers fans.

Be careful when saying things like this, because they sound very hostile.