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What is a virus?

22 Apr 2014

Viruses are prolific. There are more viruses in existence today than all the other types of living organisms in the world. All recognised life-forms are susceptible to the threat of viruses entering their cells where they exist as parasites.

It is debatable as to whether or not viruses can be described as actual living organisms. Outside of living cells viruses are wholly inert. They cannot perform any of the activities typical of ‘life’, activities such as reproduction and the breaking down and digestion of food.

The particles of a virus are so minute that they do not contain the complex apparatus required to reproduce. This is only possible with the assistance of a host. Over the course of their evolution viruses have gradually learnt to persuade certain cells within living organisms, such as humans, animals and plants, to let them in. Once inside they then hijack the cellular machinery and begin to reproduce.

Viruses have learnt to co-evolve with their hosts. As the immune systems of living organisms have developed new ways of defending against their threat, viruses seem to have consistently adapted themselves to overcome these obstacles. Viruses are also more difficult to combat with drugs than bacterial infections, as this runs the risk of actually damaging the host cell.

Structure of viruses

The structure of a virus is remarkably simple. They are made up of either ribonucleic (RNA) or deoxyribonucleic acid (DNA), which is a string of genes that contain coded instructions for producing copies of the virus. The RNA or DNA is enclosed in one or two protective shells made from proteins. These shells, or envelopes, are incredibly strong, which enables the virus to withstand the assault from the host’s immune system.

Types of viruses

Viruses vary according to how they encode their genetic information. Living organisms, including humans and other animals, bacteria, fungi, and plants, use DNA (deoxyribonucleic acid) to store their genetic code. Some viruses also store their genetic information using DNA, while others use RNA (ribonucleic acid). Viruses are therefore classified as either DNA or RNA viruses. The hepatitis C virus (HCV) is a RNA virus.

In a DNA virus there are two complementary and intertwined strands of nucleic acid (the double helix). The structure of DNA resembles two interlocking spirals, and when reproduction takes places the genetic code of the parent is correctly imposed on the child. RNA viruses, however, are made up of a series of linked strands. The consequences of this are that RNA viruses are much more unstable and so more prone to forge inexact copies of themselves. This is why so many different types of HCV have evolved.

How viruses enter the body

Viruses gain access to the human body by all possible entry routes. They are inhaled in droplets, swallowed in food and fluids, passed through saliva, passed from mother to child, through faeces, via sexual contact and through blood to blood contact.

The immune system deals swiftly with most viruses. Each mechanism of the immune system may be involved in resisting a viral attack, including white blood cells which engulf the viral particles and lymphocytes that either produce antibodies against the virus or attack virally infected cells. This allows the host to recover from most viral infections within the space of a few days or weeks. Furthermore, the immune system is often sufficiently sensitized by the infection to make a second illness from the same virus rare. However, if the virus is able to dodge or outwit the immune system, the infection can become chronic. In some cases the response of the immune system can cause as many problems as the virus. This is called an immunopathic response. This is a major issue with HCV.

Cell entry and reproduction

The surface of the virus protein envelope is studded with receptors. These receptors act as sensory organs which feed the virus with information about its surroundings. They are designed to help the virus find cells in which to establish a base. In simple terms, the receptors on the surface of the virus attach to the specific receptors on the surface of the host cell. They then confuse the host receptors into thinking that they present no danger to it. A cell can be infected by a virus only if that type of cell has a receptor site for the virus protein. Thus cold viruses infect cells in the mucous membranes of the respiratory tract, pneumonia affects lung cells and hepatitis viruses primarily infect liver cells.

Once inside a cell a virus begins to shed its protein shell. The genetic material of the virus is reproduced using substances taken from inside the cell. Each copy of the genetic material programmes the formation of a new shell. Once the new shells have formed, the new viruses are complete and are ready to leave that particular cell and infect new ones. They do this either by rupturing the cell membrane and destroying the host cell, or by slowly budding out from the surface of the membrane. It is thought that HCV achieves this by budding out.

Further reading

22 Apr 2014