Infest/ɪnˈfest/ Definition 1 of 2Pro Tip 1/3Negative Connotation Infest usually implies a situation is serious and the creatures or things are causing harm. The lake was infested with harmful algae. Pro Tip 2/3Large Numbers Use 'infest' when there are many pests or unwanted things, not just one or a few. The old barn was infested with bats. Pro Tip 3/3Undesirable Presence 'Infest' is used when the creatures or things are unwelcome and usually difficult to remove. The park has been infested with invasive species.