A virus 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. The origins of viruses in the evolutionary history of life are unclear. 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. Not all viruses cause disease, as many viruses reproduce without causing any obvious harm to the infected organism. Antiviral drugs have been developed to treat both life-threatening and more minor infections.

Read the rest of this article: http://en.wikipedia.org/wiki/Virus

_______________________________ Today's selected anniversaries:

869:

The Fourth Council of Constantinople, the eighth Catholic Ecumenical Council, was convened to discuss the patriarchate of Photios I of Constantinople. http://en.wikipedia.org/wiki/Fourth_Council_of_Constantinople_%28Roman_Catholic%29

1877:

After battling the U.S. Army for more than three months, retreating over 1,000 miles across Oregon, Washington, Idaho, and Montana, and enduring a five-day siege, Chief Joseph and his Nez Perce band finally surrendered. http://en.wikipedia.org/wiki/Chief_Joseph

1903:

Samuel Griffith became the first Chief Justice of Australia, while Edmund Barton and Richard O'Connor became the first Puisne Justices of the High Court of Australia. http://en.wikipedia.org/wiki/Samuel_Griffith

1948:

The International Union for Conservation of Nature was founded at a congress sponsored by UNESCO director Julian Huxley in Fontainebleau, France. http://en.wikipedia.org/wiki/International_Union_for_Conservation_of_Nature

1986:

The British newspaper The Sunday Times published former nuclear technician Mordechai Vanunu's story revealing details of Israel's nuclear capability. http://en.wikipedia.org/wiki/Mordechai_Vanunu

_____________________________ Wiktionary's word of the day:

inure (v): To cause to become accustomed to something unpleasant by prolonged exposure http://en.wiktionary.org/wiki/inure

___________________________ Wikiquote quote of the day:

Anyone who takes himself too seriously always runs the risk of looking ridiculous; anyone who can consistently laugh at himself does not. --Václav Havel http://en.wikiquote.org/wiki/V%C3%A1clav_Havel