Which term describes something that is harmful, likely to spread, and potentially cause destruction?

The correct term that describes something harmful, likely to spread, and potentially destructive is "malignant." This term is often used in medical contexts to refer to tumors or diseases that are aggressive and have the potential to invade nearby tissues or metastasize to other parts of the body. The connotation of malignancy goes beyond just harmfulness—it implies a serious threat to health and well-being due to its aggressive nature. This term is frequently contrasted with "benign," which refers to conditions that are not harmful or do not spread.

In this context, benign conditions typically do not pose a significant risk of causing destruction or severe harm. Innocuous suggests something that is harmless and not likely to cause any negative effects, which does not align with the criteria given in the question. Non-virulent refers to a pathogen that is not highly infectious or harmful, thus lacking the threatening characteristic outlined in the question. Therefore, "malignant" is the most accurate descriptor, as it encapsulates the notion of harm and potential spread effectively.