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