A more thorough explanation:

Definition: A toxin is a poisonous substance that can cause harm to living organisms. It can be any type of poison or toxicant. In medical terms, it refers to a diffusible alkaloidal substance that can cause harm to the body. This includes poisonous products of disease-producing bacteria, such as ptomaines, abrin, brucin, or serpent venoms.

Examples:

• Botulinum toxin - a neurotoxin produced by the bacterium Clostridium botulinum that can cause paralysis.
• Tetrodotoxin - a toxin found in pufferfish that can cause paralysis and death.
• Ricin - a toxin found in castor beans that can cause organ failure and death.

The examples illustrate how toxins can come from various sources and can cause harm to living organisms. Botulinum toxin is produced by bacteria, tetrodotoxin is found in a specific type of fish, and ricin is found in a plant. All of these toxins can cause serious harm to the body and can even be deadly.