A virus is an intrusive biological agent that reproduces inside the cells of living hosts. When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus, at an extraordinary rate. Unlike most living things, viruses do not have cells that divide; new viruses are assembled in the infected host cell. Over 2,000 species of viruses have been discovered.
The origins of viruses are unclear: some may have evolved from plasmids -- pieces of DNA that can move between cells, while others may have evolved from bacteria. A virus consists of two or three parts: genes, made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects the genes; and in some viruses, an envelope of fat that surrounds and protects them when they are not contained within a host cell. Viruses vary in shape from the simple helical and icosahedral to more complex structures. Viruses are about 1/100 the size of bacteria; it would take 30,000 to 750,000 of them, side by side, to stretch to 1 centimetre (0.39 in).
Viruses spread in many different ways. Each species of virus relies on a particular method. Plant viruses are often spread from plant to plant by insects and other organisms, known as vectors. Some viruses of animals, including humans, are spread by the transfer of body fluids which may occur by a number of means. Viruses such as influenza are spread through the air by people when they cough or sneeze. Viruses such as norovirus, are transmitted by the faecal-oral route, which contaminates hands, food and water. Rotavirus is often spread by direct contact with infected children. The human immunodeficiency virus, HIV, is one of several major viruses that are transmitted during sex. Other viruses are spread by blood-sucking insects.
Viral infections can cause disease in humans, animals and even plants. However, they are usually eliminated by the immune system, conferring lifetime immunity to the host for that virus. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity can prevent some viral infections.
Animals, including humans, have many natural defences against viruses. Some are non-specific and protect against many viruses regardless of the type. This innate immunity is not improved by repeated exposure to viruses and does not retain a "memory" of the infection. The skin of animals, particularly its surface, which is made from dead cells, prevents many types of viruses from infecting the host. The acidity of the contents of the stomach kills many viruses that have been swallowed. When a virus overcomes these barriers and enters the host, other innate defences prevent the spread of infection in the body. A special hormone called interferon is produced by the body when viruses are present, and this stops the viruses from reproducing by killing the infected cell and its close neighbours. Inside cells, there are enzymes that destroy the RNA of viruses. This is called RNA interference. Some blood cells engulf and destroy other virus infected cells.
http://en.wikipedia.org/wiki/Introduction_to_viruses
The origins of viruses are unclear: some may have evolved from plasmids -- pieces of DNA that can move between cells, while others may have evolved from bacteria. A virus consists of two or three parts: genes, made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects the genes; and in some viruses, an envelope of fat that surrounds and protects them when they are not contained within a host cell. Viruses vary in shape from the simple helical and icosahedral to more complex structures. Viruses are about 1/100 the size of bacteria; it would take 30,000 to 750,000 of them, side by side, to stretch to 1 centimetre (0.39 in).
Viruses spread in many different ways. Each species of virus relies on a particular method. Plant viruses are often spread from plant to plant by insects and other organisms, known as vectors. Some viruses of animals, including humans, are spread by the transfer of body fluids which may occur by a number of means. Viruses such as influenza are spread through the air by people when they cough or sneeze. Viruses such as norovirus, are transmitted by the faecal-oral route, which contaminates hands, food and water. Rotavirus is often spread by direct contact with infected children. The human immunodeficiency virus, HIV, is one of several major viruses that are transmitted during sex. Other viruses are spread by blood-sucking insects.
Viral infections can cause disease in humans, animals and even plants. However, they are usually eliminated by the immune system, conferring lifetime immunity to the host for that virus. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity can prevent some viral infections.
Animals, including humans, have many natural defences against viruses. Some are non-specific and protect against many viruses regardless of the type. This innate immunity is not improved by repeated exposure to viruses and does not retain a "memory" of the infection. The skin of animals, particularly its surface, which is made from dead cells, prevents many types of viruses from infecting the host. The acidity of the contents of the stomach kills many viruses that have been swallowed. When a virus overcomes these barriers and enters the host, other innate defences prevent the spread of infection in the body. A special hormone called interferon is produced by the body when viruses are present, and this stops the viruses from reproducing by killing the infected cell and its close neighbours. Inside cells, there are enzymes that destroy the RNA of viruses. This is called RNA interference. Some blood cells engulf and destroy other virus infected cells.
http://en.wikipedia.org/wiki/Introduction_to_viruses
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