IPA: /ˌɪn.fɛˈsteɪ.ʃən/
KK: /ɪnˈfɛsteɪʃən/
A situation where a large number of pests or harmful organisms are present in a specific area, often causing damage or harm.
The garden suffered from an infestation of aphids, which damaged the plants.
Infestation is formed from "in-" (meaning into) and "festare" (from Latin "festatio", meaning to feast or to consume). The word describes the act of pests entering and consuming a place, akin to a feast of unwanted creatures.
Think of pests coming 'into' ('in-') a place to 'consume' ('festare') resources, which helps you remember that infestation refers to an overwhelming presence of pests.