What kind of substance is described as extremely harmful or severe in its effects?

The term that describes a substance as extremely harmful or severe in its effects is "virulent." This word is often used in contexts such as disease, where it refers to pathogens that cause serious illness or have a strong ability to spread, indicating their dangerous nature. In chemistry or environmental science, a virulent substance could refer to a toxic chemical that poses significant health risks to humans or the ecosystem. Understanding the implications of such substances is critical in fields like law enforcement, environmental science, and public health, as it helps professionals identify and manage risks associated with exposure.

In contrast, terms like innocuous, beneficial, and harmless imply safety or positive effects, which do not align with the concept of a substance being harmful or severe.