An infestation is when large numbers of pests or unwanted creatures invade a place where they don't belong and become difficult to get rid of. When a house has a mouse infestation, it means mice have moved in, multiplied, and spread throughout the building. When a garden suffers an aphid infestation, these tiny insects cover the plants in huge numbers, feeding on them and causing damage.

The word infestation usually refers to animals we consider pests: insects like termites or bedbugs, rodents like rats, or parasites like lice. It suggests both quantity (lots of them) and persistence (they've settled in and keep coming back). A single mouse in your garage isn't an infestation, but dozens of mice living in your walls, raising families, and showing up everywhere definitely is.

People use infest as a verb: cockroaches might infest an apartment building, or weeds might infest a lawn. The word carries a sense of something unwelcome taking over a space, spreading where it shouldn't be, and requiring serious effort to eliminate. An infestation means the pests have made themselves at home and become a real problem that may need professional help to solve.