For something that's very harmful, especially a tumor that's cancerous, use the term malignant.

Malignant and its opposite benign are medical terms used to describe a tumor or growth as either cancerous or not respectively. The gn part of both words comes from the Latin word for born, differing in the prefix mal "evil" and bene "kind." A malignant tumor grows uncontrollably and spreads to other parts of the body. Though less common, malignant can also be used to mean "evil, malicious" like when someone has a malignant imagination.

Definitions of malignant

adj dangerous to health; characterized by progressive and uncontrolled growth (especially of a tumor)

relating to or affected with cancer
not dangerous to health; not recurrent or progressive (especially of a tumor)

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