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These individual virus particles are the infectious form of a virus outside the host cell. Virions, single virus particles, are very small, about 20–250 nanometers (1 nanometer = 1/1,000,000 mm). Still, it was many years before it was proven that these “filterable” infectious agents were not simply very small bacteria but were a new type of tiny, disease-causing particle. In 1892, Dmitri Ivanowski showed that this disease could be transmitted in this way even after the Chamberland-Pasteur filter had removed all viable bacteria from the extract. In 1886, Adolph Meyer demonstrated that a disease of tobacco plants, tobacco mosaic disease, could be transferred from a diseased plant to a healthy one through liquid plant extracts. Viruses were first discovered after the development of a porcelain filter, called the Chamberland-Pasteur filter, which could remove all bacteria visible under the microscope from any liquid sample. While most biological diversity can be understood through evolutionary history, such as how species have adapted to conditions and environments, much about virus origins and evolution remains unknown. They vary in their structure, their replication methods, and in their target hosts or even host cells. They do not metabolize or grow, but are assembled in their mature form. Viruses replicate, but to do so, they are entirely dependent on their host cells. Living things grow, metabolize, and reproduce. Viruses infect all forms of organisms including bacteria, archaea, fungi, plants, and animals. Instead, they infect a host cell and use the host’s replication processes to produce progeny virus particles. They have no plasma membrane, internal organelles, or metabolic processes, and they do not divide. Viruses are acellular, parasitic entities that are not classified within any domain because they are not considered alive. Modern viruses are thought to be a mosaic of bits and pieces of nucleic acids picked up from various sources along their respective evolutionary paths. No one knows exactly when viruses emerged or from where they came, since viruses do not leave historical footprints such as fossils. (credit a: scale-bar data from Matt Russell credit b: modification of work by USDA, Department of Plant Pathology Archive, North Carolina State University) (b) The leaves of an infected plant are shown.
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Describe how viruses were first discovered and how they are detected.By the end of this section, you will be able to: