Deadly poisons, sometimes carrying death to humans, are quite common on our planet. And not only in inanimate and living nature, but also in human society. Poisonous compounds are found in volcanic emissions, in some caves and reservoirs. Many plants and animals also synthesize various toxic substances that they use as protection against their enemies. Often these poisons are dangerous for humans. People have also long used poisonous substances. Moreover, for a variety of purposes: both as a means of treating various diseases, and as a deadly and invisible weapon in the struggle for political influence and wealth. Toxic substances were especially widely used in the Ancient World and the Middle Ages. However, in subsequent periods of history, they did not lose their role as quiet killers. The distribution of poisons on Earth and their use by humans is described in another book in the series.