Introduction To Viruses

Introduction To Viruses

Virus Glossary Virion

A single, fully functional virus particle outside its host cell

Capsid The protein shell surrounding the virus's genes; this shell is made from many smaller proteins which are identical and are called capsomers. Envelope

Some viruses have a bubble of fat that surrounds the virion.

Gene

A segment of DNA or RNA; genes are like sentences made of the "letters" of the nucleotide alphabet. Genes direct the reproduction of viruses. Different types of viruses have genes made from DNA or RNA but not both.

Host range

The animals, plants or bacteria that any one type of virus can infect.

Bacteriophages

The viruses that reproduce in bacteria

Cell tropism The type of cell a virus can reproduce in Serotype

A grouping of viruses based on the antigens on the surface of virus

Symmetry

All viruses of a type are identical and their particles have a cubical, helical or complex structure.

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A virus is an invasive 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 5,000 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. Just as many viruses are very specific as to which host species or tissue they attack, each species of virus relies on a particular method for propagation. 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 exposure to infected bodily fluids. Viruses such as influenza are spread through the air by droplets of moisture when people 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 transmitted by bodily fluids transferred during sex. Others, such as the Dengue virus, 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.