Viruses are biological particles on the border of the living and inanimate world, different from all other life forms. Viruses are not alive due to a lack of metabolism and cellular structures, and their inability to reproduce without host cells makes them parasites. Nevertheless, they are not dead because of the content of genetic material that can be multiplied and create new individuals. They are made up of nucleic acid, single or double helix RNA or DNA and protein envelopes. Until recently, viruses were not considered interesting, they were viewed only in the context of the causative agent of the disease, so they do not have a place on life tree. Their host circle covers all types of life. Diseases that viruses cause are: flu, measles, shrimp, rabies, polio, AIDS, and others. They can also cause tumors. New hypotheses about viruses based on the molecular biology method indicate the great age of the virus and the fact that the viruses were active promoters of the evolution of early organisms. Viruses have a large amount of unknown genes that spread to other species. Despite the diseases they cause, they have a very important impact on our immune system, terrestrial and marine ecosystems, climate regulation and the evolution of all species on Earth.