Nature of viruses

Viruses exist in two different states, the extracellular infectious particle or virion and the intracellular state consisting of viral nucleic acid.

The virion consists of a protein coat or capsid, which encloses a genome of either RNA or DNA. The entire structure is called the nucleocapsid.

The capsid may be a polyhedron or a helix, or a combination of both (in some phages). Viruses are infective micro­organisms that show several differences from typical microbial cells.
They show both animal and plant characteristics so they cannot be classified as any particular group.

history of viruses

After the discovery by Louis Pasteur and Robert Koch that infectious diseases were caused by minute living organisms or 'germs', it was expected that the germs for all infectious diseases would be discovered. However, bacteriological techniques failed to demonstrate the causative organisms for many diseases like measles, small pox, rabies and mumps.

The Russian botanist Ivanovski (lwanowsky) (1892) was the first to give clear cut evidence of a virus. Mayer (1886) had demon­strated that when juice from tobacco plants infected with the 'mosaic' disease was injected into healthy plants, it reproduced the mosaic disease. Boiling the juice destroyed the infectivity. Mayer thought that, the causative agent was a bacterium. Inoculation of tobacco plants with a variety of bacteria, however, failed to produce the tobacco, mosaic disease.
Ivanovski confirmed the observations of Mayer, and also made another very important one. Even after filtering through the finest bacterial filters, the juice still remained infective. Ivanovski concluded that the agent was smaller than any known bacterium, but he still considered it to be a bacterium. This agent was later called a virus. Bacteriophages ( viruses that parasitise bacteria) were discovered by the French scientist d'Herelle (1917), who found that some agent was destroying his cultures of bacilli.

Schelsinger (1933) was the first to determine the composition of a virus. He showed that a bacteriophage consists of only protein and DNA.

In 1935 Stanley crystallized the virus causing tobacco mosaic disease, and demonstrated that the crystals retained their infectivity when inoculated into healthy plants. He thus showed that viruses were not like typical cells.

In 1952 Hershey and Chase studied - the T2 bacteriophage and demonstrated that (1) the genetic information is carried in the phage DNA, and that (2) infection is the result of penetration of viral DNA into cells.

Introduction

any member of a unique class of infectious agents, which were originally distinguished by their smallness (hence, they were described as 'filtrable' because of their ability to pass through bacteria-retaining filters) and their inability to replicate outside of a living host cell; because these properties are shared by certain bacteria (rickettsiae, chlamydiae), viruses are further characterized by their simple organization and their unique mode of replication. A virus consists of genetic material, which may be either DNA or RNA, and is surrounded by a protein coat and, in some viruses, by a membranous envelope.

For a list of animal viruses and their classification see Table 8.1.

Unlike cellular organisms, viruses do not contain all the biochemical mechanisms for their own replication; viruses replicate by using the biochemical mechanisms of a host cell to synthesize and assemble their separate components. When a complete virus particle (virion) comes in contact with a host cell, the viral nucleic acid and, in some viruses, a few enzymes are introduced into the host cell.

Viruses vary in their stability; some such as poxviruses, parvoviruses and rotaviruses are very stable and survive well outside the body while others, particularly those viruses that are enveloped, such as herpesvirus, influenza virus, do not survive well and therefore usually require close contact for transmission and are readily destroyed by disinfectants, particularly those with a detergent action. Some viruses produce acute disease while others, sometimes referred to as slow viruses, such as retroviruses and lentiviruses and the scrapie agent, produce diseases which progress often to death over many years. Viruses in several families are transmitted by arthropod vectors.