Exercise 4. Read and translate the text.

Virology

In 1892 Dimitri Ivanovski showed that tobacco mosaic disease could be transmitted by extracts that were passed through filters fine enough to exclude even the smallest known bacteria.

The existence of viruses that infect bacteria (bacteriophages) was first recognized by Frederick Twort in 1911, and, independently, by Felix d'Herelle in 1917. As bacteria could be grown easily in culture, this led to an explosion of virology research.

Since 1938 electron microscopy has made it possible to see virus particles, and since 1945 the possibility of preparing ultra fine microscopic sections has furthered the study of the development of the virus in tissues.

Now virology is the branch of medicine studying viruses and virus-like agents: their structure, classification and evolution, their ways to infect and exploit cells for virus reproduction, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy.

Virology is often considered to be a part of microbiology or of pathology.

A major branch of virology is virus classification. Viruses can be classified according to the host cell they infect: animal viruses, plant viruses, fungal viruses, and bacteriophages (viruses infecting bacteria, which include the most complex viruses).

The most widely used classification system distinguishes viruses according to the type of nucleic acid they use as genetic material and the viral replication method they employ to coax host cells into producing more viruses:

· DNA viruses

· RNA viruses

· everse transcribing viruses

In addition virologists also study ''subviral particles'': viroids, satellites and prions.

Viruses continue to be investigated because they are held to be possible causative agents of some human cancers. Viruses can have high rates of mutations that keep them undetectable.

POST-TEXT ASSIGNMENTS

Exercise 5. Answer the questions:

1. Who showed that there exist the microorganisms smaller than any known bacteria?

2. What discoveries were made in 1911 and 1917?

3. What has made it possible to see virus particles?

4. What does modern virology study about viruses?

5. What is the major branch of virology?

6. How can viruses be classified according to the host cell they infect?

7. What is the most widely used classification of viruses?

8. What types of viruses are distinguished according to this classification?

9. What keeps viruses being undetectable?

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