Martinus Beijerinck in the context of "Virus (biology)"

Delft (Dutch pronunciation: [ˈdɛl(ə)ft] ) is a city and municipality in the province of South Holland, Netherlands. It is located between Rotterdam, to the southeast, and The Hague, to the northwest. Together with them, it is a part of both the Rotterdam–The Hague metropolitan area and the Randstad.

Delft is a popular tourist destination in the Netherlands, famous for its historical connections with the reigning House of Orange-Nassau, for its blue pottery, for being home to the painter Jan Vermeer, and for hosting Delft University of Technology (TU Delft). Historically, Delft played a highly influential role in the Dutch Golden Age. In terms of science and technology, thanks to the pioneering contributions of Antonie van Leeuwenhoek and Martinus Beijerinck, Delft can be considered to be the birthplace of microbiology.

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, more than 16,000 of the millions of virus species have been described in detail. The study of viruses is known as virology, a subspeciality of microbiology.

When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent viral particles, or virions, consisting of (i) genetic material, i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria.

Virology is the scientific study of biological viruses. It is a subfield of microbiology that focuses on their detection, structure, classification and evolution, their methods of infection and exploitation of host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy.

The identification of the causative agent of tobacco mosaic disease (TMV) as a novel pathogen by Martinus Beijerinck (1898) is now acknowledged as being the official beginning of the field of virology as a discipline distinct from bacteriology. He realized the source was neither a bacterial nor a fungal infection, but something completely different. Beijerinck used the word "virus" to describe the mysterious agent in his 'contagium vivum fluidum' ('contagious living fluid'). Rosalind Franklin proposed the full structure of the tobacco mosaic virus in 1955.

Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economic importance to humans varies widely. A recent metatranscriptomics study suggests that the host diversity of viroids and viroid-like elements is broader than previously thought and is not limited to plants, encompassing even the prokaryotes.

The first discoveries of viroids in the 1970s triggered the historically third major extension of the biosphere—to include smaller lifelike entities—after the discoveries in 1675 by Antonie van Leeuwenhoek (of the "subvisible" microorganisms) and in 1892–1898 by Dmitri Iosifovich Ivanovsky and Martinus Beijerinck (of the "submicroscopic" viruses). The unique properties of viroids have been recognized by the International Committee on Taxonomy of Viruses, in creating a new order of subviral agents.

The history of virology – the scientific study of viruses and the infections they cause – began in the closing years of the 19th century. Although Edward Jenner and Louis Pasteur developed the first vaccines to protect against viral infections, they did not know that viruses existed. The first evidence of the existence of viruses came from experiments with filters that had pores small enough to retain bacteria. In 1892, Dmitri Ivanovsky used one of these filters to show that sap from a diseased tobacco plant remained infectious to healthy tobacco plants despite having been filtered. Martinus Beijerinck called the filtered, infectious substance "contagium vivum fluidum." Although the particles could not been seen yet, his discovery is considered to be the beginning of virology.

The subsequent discovery and partial characterization of bacteriophages by Frederick Twort and Félix d'Herelle further catalyzed the field, and by the early 20th century many viruses had been discovered. In 1926, Thomas Milton Rivers defined viruses as obligate parasites. Viruses were demonstrated to be particles, rather than a fluid, by Wendell Meredith Stanley, and the invention of the electron microscope in 1931 allowed their complex structures to be visualised.

Contagium vivum fluidum (Latin: "contagious living fluid") was a phrase first used to describe a virus, and underlined its ability to slip through the finest ceramic filters then available, giving it almost liquid properties. Martinus Beijerinck (1851–1931), a Dutch microbiologist and botanist, first used the term when studying the tobacco mosaic virus, becoming convinced that the virus had a liquid nature.

The word "virus", from the Latin for "poison", was originally used to refer to any infectious agent, and gradually became used to refer to infectious particles. Bacteria could be seen under microscope, and cultured on agar plates. In 1890, Louis Pasteur declared "tout virus est un microbe": "all infectious diseases are caused by microbes".