A chemical, physical, or biological agent that causes disease or some alteration of the normal structure and function of an organism. Usually refers to poisonous substance produced during the metabolism and growth of certain microorganisms and some higher plant and animal species, or any poisonous isomer, homolog, or derivative of such a substance. It causes either permanent or reversible injury to the health of a living thing on contact or absorption, typically by interacting with biological macromolecules such as enzymes and receptors. Onset of effects may be immediate or delayed, and impairments may be slight or severe. (NCI)