What is a virus

What is a virus?

A virus is a microorganism that invades living cells and uses their chemical machinery to keep itself alive and to replicate itself causing diseases such as hepatitis C.

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 single 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 hepatitis C 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 with most viruses swiftly. 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 sensitised 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 hepatitis C.

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 this is the case for hepatitis C. A liver cell normally replicates once every 12-18 months, a hepatitis C virus forces the cell to replicate up to 50 times a day until it is damaged or destroyed.