A toxin (Greek: τοξικόν, toxikon) is a poisonous substance produced by living cells or organisms. (Although technically man is a living organism, man-made substances created by artificial processes usually aren't considered toxins by this definition.)

For a toxic substance not produced by living organisms, "toxicant" is the more appropriate term, and "toxics" is an acceptable plural.

Toxins can be small molecules, peptides, or proteins that are capable of causing disease on contact with or absorption by body tissues interacting with biological macromolecules such as enzymes or cellular receptors. Toxins vary greatly in their severity, ranging from usually minor and acute (as in a bee sting) to almost immediately deadly (as in botulinum toxin).

For more information about Toxin, read the full article at Wikipedia.
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