A virus is a non-living biochemical structure. It does not carry out any of the normal life processes and is incapable of doing anything unless it invades a living cell. The virus is essentially a cell hijacker, it invades the cell and uses its nucleic acid, either DNA or RNA according to the type of virus, to take over the protein manufacturing mechanism of the cell. The cell’s resources are diverted from self-maintenance to production of new virus particles. The cell will eventually lyse (burst) and release the new viral particles which drift about until they contact new cells which they can invade. The process of viral production is called replication, and the cycle of cell invasion then bursting is called the lytic cycle.

A virus can be damaging to our system due to the rapid death of cells in the tissues invaded, and through the release of toxins produced within the infected cells which are released as the cell lyses.

A virus which attacks bacteria is given the name bacteriophage, because it has to contact the extremely tiny hosts it has arm like proteins with recognition molecules on their ends, this increases the possibility of contact with a host bacterium. The contractile neck acts like a syringe, the spiral protein making it up can contract to inject the viral nucleic acids through the double outer membrane of the bacterial cell.

Typical virus particles attacking eukaryote cells are simpler in design having a protein coat with recognition proteins at intervals around the surface. On contact with a suitable host cell the cell membrane is opened and the viral nucleic acid enters the cytoplasm. The virus can now take two possible pathways, the lytic cycle as outlined previously, or lysogeny. The latter course of action involves incorporating its nucleic acid into the main chromosome of the host cell. Once incorporated it may lie dormant as a ‘prophage’ for many years. When conditions are suitable the prophage is extracted from the host DNA and then proceeds to enter the lytic cycle whereby it replicates itself and gets released to infect other cells.

The dormant prophages can cause repeated episodes of damage over many years. The cold-sore virus is a good example of this, it lies dormant in brain cells and then moves down the neurons to the original site of infection and enters the lytivc cycle to cause a new cold sore and thus increase its chances of being passed to a new host.

Some viruses contain RNA instead of DNA, examples include Influenza, Poliomyelitis,Measles, Mumps and HIV). These invade the cell and the RNA is reverse transcribed into DNA which can then take part in lysogeny. The sites at which the retro-virus cuts the host DNA and attaches are often referred to as oncogenes. The retrovirus needs to contain the enzyme reverse transcriptase which catalyses the production of DNA from the code in the viral RNA (the reverse of the normal transcription process). The viral DNA may then be incorporated into the host DNA.

The virus is spread by droplet infection, the infected individual is induced to sneeze due to the irritation of the nasal and tracheal passages resulting from the lysis of epithelial cells infected with the virus. The droplets are breathed in by a new host. If a sufficient dose of viruses are taken in then some may eascape the natural defences of our mucous and white blood cells to invade epithelial cells. These will pass through the lytic cycle and release huge numbers of viral particles which will invade fresh epithelial cells. The toxins released from the damaged cells will result in a high temperature and muscle and joint pains. The infected human will usually begin to produce antibodies against the virus after a few days and these will attach to the recognition proteins on the virus surface and thereby prevent the entry of the virus into the cells. By this time the person will have distributed huge numbers of viruses into the atmosphere in the droplets released during bouts of coughing and sneezing. There are a number of strains of the influenza virus and immunity developed against one strain offers no protection against the other strains, this means that one can catch a whole sreies of different flu’s one after the other. Influenza is a serious illness and new strains can cause considerable numbers of deaths especially in the very young or old. There is no treatment available which will prevent the disease’s progress as viruses are not alive and therefore cannot be killed by antibiotics. Only the symptoms can be alleviated with anti-inflammatory pain killers such as aspirin. The disease tends to spread more in the winter months because of our tendency to close windows to prevent draughts, this causes a vast increase in airborne viral particles in the enclosed space and this leads to a more effective transfer to new hosts.

Human immune deficiency virus is a retrovirus affecting the T-helper cells of our immune system. Once infected the host cells are killed off and since they are our means of activating antibody production by the b-lymophocytes our immune system fails. Secondary, opportunistic organisms or cancers then cause fatal diseases. The incubation period is very variable and can be as long as 8 years. The virus is only passed on by contact with body fluids, such as semen or blood and can only enter our system through damaged surfaces. It has a large number of strains which readily mutate and this makes production of a vaccine unlikely. Treatment is by use of drugs which reduce RNA production or otherwise block viral replication. The treatment does not remove the infection just increases the time taken for AIDS to develop (ie loss of the immune system).