Viruses are prolific. There are probably more viruses in existence than the number of types of living organisms in the world. All recognised life-forms have to deal with viruses living as parasites.
It is matter of debate about whether viruses are actually truly living organisms. Outside of living cells viruses are wholly inert: they cannot perform any of the activities typical of life such as reproduction and breaking down and digesting food.
The particles of a virus are so small that they cannot contain the complex apparatus required to reproduce themselves. They need assistance from a host. What they have learnt to do over years of evolution is to persuade certain cells within living organisms to let them in and then once inside they hijack the cellular machinery to reproduce themselves.
Viruses have co-evolved with their hosts, so every time our immune system has developed a way of combating a virus, the virus adapts and comes up with a new function of its own to counteract our defences. Viruses are also more difficult to combat with drugs than bacterial infections because of the risk of damaging the host cell.
Structure of Viruses
Their structure is very simple. They are made up of ribonucleic (RNA) or deoxyribonucleic acid (DNA), which is a string of genes that contains coded instructions for making copies of the virus, enclosed in one or two protective shells which are made of proteins. These shells or envelopes must be very strong to withstand the assault of the immune system of the host.
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 encode their genetic information as DNA, while others use RNA (ribonucleic acid). Viruses are therefore classified as DNA viruses or RNA viruses. HCV is a RNA virus.
In DNA viruses there are two complementary intertwined strands of nucleic acid (the double helix). The structure of DNA is like two interlocking spirals. This means that 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 consequence of this is that RNA viruses are much more unstable and so more likely to make inexact copies of themselves. This is why so many different types of the hepatitis C virus 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 systems deals swiftly with most viruses. Each mechanism of the immune system may be involved in resisting a viral attack including white blood cells that engulf the viral particles and lymphocytes that produce antibodies against the virus or attack virally infected cells. This leads to recovery from most viral infections within a few days to 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. It is a major issue with HCV.
Cell entry and reproduction
The surface of the virus protein envelope or protective coat is studded with receptors. These receptors are in fact sensory organs that tell the virus 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 confuse the host receptors into thinking that they present no danger. 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 infect liver cells. When viruses are present in the body the immune system will try to eliminate them and usually it will succeed.
When inside a cell a virus sheds 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 protein shell. Once the shells have formed the new viruses are complete and they are ready to leave the cell to infect new cells. They do this by either rupturing the cell membrane which destroys the host cell or by slowly budding out from the surface of the cell membrane. It is thought that HCV does it by budding out.
