Pestilentially refers to something that is harmful or destructive, particularly in a contagious or infectious way. If you're looking for some synonyms to describe this type of situation, some options include infectiously, contagiously, noxiously, malignantly, deleteriously, toxicologically, lethal, and fatally. Each of these words emphasizes the danger and harm that can come from exposure to something harmful. Additionally, words like nocuously and malignantly suggest intentional harm or malice, while terms like toxicologically and fatally highlight the potential for serious and dangerous consequences as a result of exposure to certain substances or conditions.