Related topics: bacteria · influenza · immune system · protein · virus

The rise, fall and resurgence of gene therapy

Twenty years ago, scientists and investors were giddy over the promise of gene therapy to cure a host of deadly diseases. But the tragic death of a young man from an experimental treatment in 1999 nearly ended a scientist's ...

Bat influenza viruses possess an unexpected genetic plasticity

Bat-borne influenza viruses enter host cells by utilizing surface exposed MHC-II molecules of various species, including humans. Now, an international research team from Germany (Medical Center—University of Freiburg and ...

A new methodology for sequencing viruses

NUS scientists have developed a more efficient method to sequence the complete genomes of infectious diseases carried by mosquitoes directly from patient samples.

Detailed map shows how viruses infect humans

Biologists at Columbia University Vagelos College of Physicians and Surgeons have leveraged a computational method to map protein-protein interactions between all known human-infecting viruses and the cells they infect. The ...

Technique combats widespread passion fruit disease

Passion fruit woodiness caused by cowpea aphid-borne mosaic virus (CABMV), the disease that most affects passion fruit (Passiflora edulis) grown in Brazil, can be treated with a relatively simple technique, according to a ...

Plant protection: Researchers develop new modular vaccination kit

It could become significantly easier to vaccinate plants against viruses in the future. Scientists at Martin Luther University Halle-Wittenberg (MLU), the Leibniz Institute of Plant Biochemistry (IPB) and the National Research ...

Analyzing genomes to improve disease control in poultry

Marek's disease—a highly contagious viral disease caused by a herpesvirus—is a constant threat to poultry worldwide. It is also one of the most preventable diseases with vaccination. However, while vaccines prevent poultry ...

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Virus

I: dsDNA viruses II: ssDNA viruses III: dsRNA viruses IV: (+)ssRNA viruses V: (−)ssRNA viruses VI: ssRNA-RT viruses VII: dsDNA-RT viruses

A virus (from the Latin virus meaning toxin or poison) is a microscopic infectious agent that can reproduce only inside a host cell. Viruses infect all types of organisms: from animals and plants, to bacteria and archaea. Since the initial discovery of tobacco mosaic virus by Martinus Beijerinck in 1898, more than 5,000 types of virus have been described in detail, although most types of virus remain undiscovered. Viruses are ubiquitous, as they are found in almost every ecosystem on Earth, and are the most abundant type of biological entity on the planet. The study of viruses is known as virology, and is a branch of microbiology.

Viruses consist of two or three parts: all viruses have genes made from either DNA or RNA, long molecules that carry genetic information; all have a protein coat that protects these genes; and some have an envelope of fat that surrounds them when they are outside a cell. Viruses vary in shape from simple helical and icosahedral shapes, to more complex structures. They are about 1/100th the size of bacteria. 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.

Viruses spread in many ways; plant viruses are often transmitted from plant to plant by insects that feed on sap, such as aphids, while animal viruses can be carried by blood-sucking insects. These disease-bearing organisms are known as vectors. Influenza viruses are spread by coughing and sneezing, and others such as norovirus, are transmitted by the faecal-oral route, when they contaminate hands, food, or water. Rotaviruses are often spread by direct contact with infected children. HIV is one of several viruses that are transmitted through sexual contact.

Not all viruses cause disease, as many viruses reproduce without causing any obvious harm to the infected organism. Viruses such as hepatitis B can cause life-long or chronic infections, and the viruses continue to replicate in the body despite the hosts' defence mechanisms. In some cases, these chronic infections might be beneficial as they might increase the immune system's response against infection by other pathogens. However, in most cases viral infections in animals cause an immune response that eliminates the infecting virus. These immune responses can also be produced by vaccines that give lifelong immunity to a viral infection. Microorganisms such as bacteria also have defences against viral infection, such as restriction modification systems. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat both life-threatening and more minor infections.

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