IPA: /pɛsˈtɪf.ə.rəs.li/
KK: /pɛsˈtɪfərəsli/
In a way that causes or spreads disease or harm; harmful or dangerous.
The pests infested the garden pestiferously, damaging the plants.
Pestiferously → It is formed from "pestifer" (from Latin "pestifer", meaning bringing disease) and the adverbial suffix "-ously" (meaning in a manner of). The word "pestiferously" means in a manner that brings disease or harm.
Think of something that brings disease ('pestifer') and remember that 'pestiferously' describes doing something in a harmful way.