Viruses are the simplest and smallest living creatures, and they also have the strangest system of microbial reproduction. Carl Linneaus, a great Swedish biologist of the 18th century, author of the system of scientific terms currently used to classify living beings, divided nature into three kingdoms: animal, vegetable and mineral, but he was unaware of the existence of viruses. Viruses live only inside other beings, plants and animals, and even some (called bacteriophages by them) do so inside bacteria. We have said they live when, in reality, this is debatable, since they do not present any of the particularities inherent to what we call life. 

The existence of viruses is consecrated exclusively in the production of new ones; viruses for this invade living cells, which end up destroying them. Once inside the cell, they exercise their control over the nucleus, removing it from its normal function and using it for the production and coupling of new viruses. These microorganisms are responsible for a complex range of diseases of various severities; in humans, for example, the common cold and smallpox are caused by viruses.   

Viruses have been defined as living chemical substances because it is often difficult to distinguish them from simple chemical substances; it is even possible to crystallize some of the smaller ones, such as the cause of the tobacco mosaic disease, which attacks the leaf of the plant, from a dissolution, as if they were crystals of common salt. However, this hard treatment does not significantly reduce the virulence of the organism; if we dissolve it again and inject it into a healthy tobacco plant, it contracts the mosaic disease.