A toxin is a harmful substance produced by living organisms, such as bacteria, plants, or animals. These substances can cause damage to cells, tissues, or entire organisms when they enter the body. Toxins can be found in various forms, including proteins, peptides, and small molecules, and they often disrupt normal biological processes. Toxins can be classified into two main categories: endotoxins, which are part of the cell structure of certain bacteria, and exotoxins, which are secreted by bacteria into their environment. Understanding toxins is crucial for fields like toxicology, medicine, and environmental science, as they can have significant impacts on health and ecosystems.