A biological toxin is a harmful substance produced by living organisms, such as bacteria, plants, or animals. These toxins can cause illness or death in other organisms, including humans. Examples include botulinum toxin from the bacterium Clostridium botulinum and ricin from the castor bean plant. Biological toxins can affect various systems in the body, leading to symptoms like nausea, paralysis, or respiratory failure. They are often used in research and medicine, such as in botox treatments for wrinkles, but can also pose significant risks if encountered in nature or through contaminated food.