Toxins are substances created by plants and animals that are poisonous to humans. Toxins also include some medicines that are helpful in small doses, but poisonous in large amounts.
Most toxins that cause problems in humans come from germs such as bacteria. For example, the symptoms of cholera are caused by a toxin made by cholera bacteria.
Other toxins that cause problems include metals, such as lead, and certain chemicals in the environment.
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