page_title: Introduction to viruses
section_title: Discovery
hierarchical_section_title: Introduction to viruses / Discovery
caption_attribution_description: English: Scanning electron micrograph of HIV-1 budding from cultured lymphocyte. This image has been colored to highlight important features; see PHIL 1197 for original black and white view of this image. Multiple round bumps on cell surface represent sites of assembly and budding of virions.
context_page_description: A virus is a tiny infectious agent that reproduces inside the cells of living hosts. When infected, the host cell is forced to rapidly produce thousands of identical copies of the original virus. Unlike most living things, viruses do not have cells that divide; new viruses assemble in the infected host cell. But unlike simpler infectious agents like prions, they contain genes, which allow them to mutate and evolve. Over 4,800 species of viruses have been described in detail out of the millions in the environment. Their origin is unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria.
  Viruses are made of either two or three parts. All include genes. These genes contain the encoded biological information of the virus and are built from either DNA or RNA. All viruses are also covered with a protein coat to protect the genes. Some viruses may also have an envelope of fat-like substance that covers the protein coat, and makes them vulnerable to soap. A virus with this "viral envelope" uses it—along with specific receptors—to enter a new host cell.
context_section_description: In 1884, French microbiologist Charles Chamberland invented the Chamberland filter (or Chamberland–Pasteur filter), that contains pores smaller than bacteria. He could then pass a solution containing bacteria through the filter, and completely remove them. In the early 1890s, Russian biologist Dmitri Ivanovsky used this method to study what became known as the tobacco mosaic virus. His experiments showed that extracts from the crushed leaves of infected tobacco plants remain infectious after filtration.
  At the same time, several other scientists showed that, although these agents (later called viruses) were different from bacteria and about one hundred times smaller, they could still cause disease. In 1899, Dutch microbiologist Martinus Beijerinck observed that the agent only multiplied when in dividing cells. He called it a "contagious living fluid" (Latin: contagium vivum fluidum)—or a "soluble living germ" because he could not find any germ-like particles. In the early 20th century, English bacteriologist Frederick Twort discovered viruses that infect bacteria, and French-Canadian microbiologist Félix d'Herelle described viruses that, when added to bacteria growing on agar, would lead to the formation of whole areas of dead bacteria. Counting these dead areas allowed him to calculate the number of viruses in the suspension.
  The invention of the electron microscope in 1931 brought the first images of viruses. In 1935, American biochemist and virologist Wendell Meredith Stanley examined the tobacco mosaic virus and found it to be mainly made from protein. A short time later, this virus was shown to be made from protein and RNA. A problem for early scientists was that they did not know how to grow viruses without using live animals. The breakthrough came in 1931, when American pathologists Ernest William Goodpasture and Alice Miles Woodruff grew influenza, and several other viruses, in fertilised chickens' eggs. Some viruses could not be grown in chickens' eggs. This problem was solved in 1949, when John Franklin Enders, Thomas Huckle Weller, and Frederick Chapman Robbins grew polio virus in cultures of living animal cells. Over 4,800 species of viruses have been described in detail.
caption: Scanning electron micrograph of HIV-1 viruses, coloured green, budding from a lymphocyte
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