Viruses

virus - вирус

cell - клетка

host – хозяин, питающий паразитов организм

helical – спиралевидный

icosahedral – икосаэдральный; двадцатигранный

plasmid - плазмид

confer – давать

A virus is a 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.

A virus consists of two or three parts: genes, made from either DNA or RNA, long molecules that carry the 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 100 times smaller than 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. Plant viruses are often spread from plant to plant by insects and other organisms, known as vector s. Some viruses of animals are spread by blood-sucking insects. Each species of virus relies on a particular method. Whereas viruses such as influenza are spread through the air by people when they cough or sneeze, others such as norovirus, which are transmitted by the faecal-oral route, contaminate 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. The origin of viruses is unclear: some may have evolved from plasmids—pieces of DNA that can move between cells, while others may have evolved from bacteria.

Viral infections often cause disease in humans and animals, 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.

Answer the questions:

1. What isa virus?

2. Do viruses have cells that divide?

3. How many species of viruses have been discovered?

4. What parts does a virus consist of?

5. Do viruses vary in shape?

6. Are viruses smaller or bigger than bacteria?

7. Do viruses spread in many different ways?

8. How are plant viruses often spread?

9. How are some viruses of animals spread?

10. What do viral infections often cause in humans and animals?

11. Do antibiotics have effect on viruses?

12. Antiviral drugs have been developed to treat life-threatening infections, haven’t they?

13. Can vaccines prevent some viral infections?