section_path: Diversity/Viruses
section_name: Viruses
paragraph_text: Viruses
  
  Viruses are submicroscopic infectious agents that replicate inside the cells of organisms. Viruses infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. More than 6,000 virus species have been described in detail. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.
  
  The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids —pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. Because viruses possess some but not all characteristics of life, they have been described as "organisms at the edge of life", and as self-replicators.