Toxin

A toxin (from Ancient Greek: τοξικόν  toxikon ) is a poisonous substance produced within living cells or organisms; synthetic toxicants created by artificial processes are thus excluded.

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 (rash, itching, dermal irritation) to almost immediately fatal.