To infest means to spread throughout and occupy a place in large, unwanted numbers. When rats infest an old building, they live there, reproduce, and take over multiple rooms and walls, establishing themselves as unwelcome occupants. When weeds infest a garden, they grow so thick they crowd out the flowers and vegetables you actually wanted.

The word always suggests something unpleasant and hard to control. You'd never say butterflies infest a meadow or that laughter infests a playground. Infest is reserved for pests, parasites, or problems: cockroaches that infest a kitchen, lice that infest someone's hair, or invasive zebra mussels that infest a lake.

Notice the feeling of being overwhelmed and invaded. One ant on your counter isn't an infestation. But when hundreds of ants create trails across your kitchen and into your pantry, marching in organized lines no matter how many you wipe away, that's when people say their house is infested. An infestation means the problem has spread beyond easy control, requiring serious effort to eliminate.