Earlier this month the World Health Organisation classified the H1N1 Swine flu outbreak as a pandemic indicating the highest level of international infection risk.

This indicated that we are now experiencing an outbreak occurring over a large geographical area which is likely to have global consequences.

Many viruses are endemic in populations. The viruses freely circulate in the community and sporadic cases occur in susceptible individuals who come into contact with other infected individuals.

Many childhood viral infections are of this nature and each year the arrival of a new year of non immune children – particularly in schools- allows transmission of the virus whilst older children and adults are, by and large, immune.

A classic example of this is Herpes Zoster or Chickenpox. Many of these virus illnesses are relatively mild and such illnesses can only be irradicated by the widespread immunisation of young children.

This is the basis of the MMR vaccination program although high levels of vaccination such as those obtained by the Smallpox program are required to ensure global immunity and virus irradication.

There are large numbers of respiratory viruses which circulate mainly in the winter months when transmission rates are higher due to more time being spent indoors.

Some of these viruses are endemic and generally circulate around whereas other viruses such as the influenza viruses can mutate or change into new varieties which can suddenly infect large numbers of both children and adults and become an epidemic. These viruses tend to originate in developing countries where higher concentrations of humans, often with closer contact with domestic animals such as pigs or poultry, can result in the cross over of new strains. The epidemics then spread by contact of susceptible individuals.

Attempts can be made to isolate epidemic outbreaks by identifying affected individuals and their contacts, isolating them and in some situations using anti-viral agents to reduce infection or symptoms of infection. Initial identification of new virus outbreaks is not easy as symptoms are often non specific.

The current Swine flu outbreak was identified due to the recognition of a new contagious influenza-type illness which particularly affected younger people (aged 5 – 45) and clearly spread had already occurred when this had become apparent.

Once the new strain has been identified serological tests can be developed to confirm infection and a new vaccine can be prepared.

It then takes up to six months for the multiplication process of vaccine production to produce enough vaccine for effective population use.

Therefore each year the conventional influenza vaccine is prepared from the three most likely strains which are emerging in the Far East in preparation for the Northern hemisphere winter.

It is clear that with the current rapid spread of Swine Flu, there is unlikely to be sufficient time for this process.

The initial attempts to isolate epidemics in Mexico and USA have failed and it is believed that the H1N1 virus will soon be freely circulating in the population.

During the Bird-flu scare, there was concern that an epidemic would spread due to the mobility of the wild bird population but it was international travel by humans which brought the swine flu to UK, followed by local transmission.

Last week we were getting reports of 150 new cases per day in the UK. This week there is 10x this number and soon it may be 10x again.

True pandemic conditions may not occur until the autumn or winter.

It is expected that 50 per cent of the population may experience symptoms, 4 per cent of those affected may require hospitalisation and 1 per cent may die of the complications of infection. Clearly medical services are at risk of being overloaded and normal business activity could be severely affected due to absenteeism.

At this point specific tests and preventative anti-viral treatments are no longer practical and antiviral treatment might only be offered to at risk patients who are more likely to develop these complications.

While swine flu seems to be highly infectious, its affects seem to be less severe than originally feared and for many people, it is likely that the usual advice of rest, symptomatic relief with medication such as paracetamol, and adequate fluids will be appropriate, but with all viral illnesses, there will be a small percentage of individuals who will develop potentially life threatening complications.

I, for one, am keeping my fingers crossed, like I do whenever I board an aircraft and hope that the wings, engines, staff and passengers are all healthy.