The invasion: the viruses' deadly programme

Packing a punch: Avian flu viruses are absolutely miniscule - spiky balls of proteins about 100 nanometers across, which, if successfully transmitted into the body, will provoke a nasty infection. This strategy ensures the viruses' survival, as they are unable to multiply alone. In order to do this, the tiny foreign bodies must board a cell and exploit the existing tools for production. To help them force their way into the cells, the viruses have special proteins on their outer surfaces, the most important being Haemaglutin (H) and Neuraminidase (N). Once inside, they begin their all-conquering programme: the viruses smuggle their genetic material, which is present in the form of ribonucleic acid (RNA), into the core of the host cell, which then goes on to work on behalf of the virus. As the cell does not recognise the invader, blueprints from the viruses' proteins are constructed and then implemented in the cell's own protein production plant. In addition, eight pieces of RNA are used to build the complete genetic material of the virus, then, together with the proteins produced for the virus’ shell, they are taken to the cell's surface. There, a piece of cell membrane detaches itself from the outside of the cell, and a new virus particle emerges. This process continues with every available cell, until they’ve all been killed.

Flu viruses: Pretty, but with a nasty bite. Their spiky surfaces are formed by the viral proteins Haemaglutin and Neuraminidase.

Source: Novartis Behring

H5N1 - surface proteins give the flu virus its name

In humans, the 'real' flu, also known in the specialist jargon as Influenza, and which is more dangerous than colds, can be brought about in humans by the influenza flu types A and B (Type C also exists, although this is not responsible for serious infections). Whereas type B is only infectious for humans, type A is found most of all in water-dwelling birds and has many subtypes, of which a few are also highly infectious for humans. In order to distinguish the many different subtypes, experts orient themselves using the surface proteins. There exist 15 different variants of Haemaglutins and nine of Neuramindase for Influenza A viruses. Depending on which variant is found on the surface of the pathogen, there are given own labels, such as H1N1 and H9N2. In the meantime, the actual pathogen which is responsible for propelling Avian Flu to its grim celebrity, is known as H5N1.

Extreme mutating ability secures viruses' survival

So why are flu viruses so deadly? The problem is down to their incredible ability to mutate and adapt. Whereas with other viruses, chickenpox for example, you are infected once and, from this, will develop an immunity for life, no matter how often you are later exposed the virus, with the flu, you will always be vulnerable to a fresh infection. The virus cleverly outwits the human own defence mechanisms, which would normally protect us from a repeated infection with the same virus. However, because the flu virus changes its appearance so often, the body is no longer able to recognise it as familiar. The changes that the viruses undergo are sometimes continuous and sometimes erratic, labelled as Drift and Shift by experts, respectively.

Genetic drift - little errors in the copying machine

The genetic drift is caused by the following 'perfect' imperfection: The virus forces the host cell to copy its genetic material thousands of times. However, the copying process is imperfect. In every perhaps 10,000th or 100,000th newly manufactured copy, a tiny error will creep in. This is because the enzyme which copies the RNA is not quite as good at its job as it should be, and as a result of this quirk, the genetic material of the virus undergoes a miniscule but gradual change, in which the proteins on the surface of the virus will also occasionally change. This prevents the immune system from being able to recognise the invaders and from mounting a quick and effective attack. The overall effect of this is that repeated, regionally focused (endemic) waves of flu infection, which might possibly spread further and cause an epidemic, are able to occur. However, as the virus has usually only undergone a miniscule mutation, the bulk of the population retain at least partial immunity, preventing a border- and population-hopping spread of the avian flu virus - a much-feared pandemic.

Genetic shift: Exchanging whole pieces of the genome

The flu virus has yet another, much more dangerous strategy up its sleeve. With the so-called Shift, the virus is able to swap entire sections of its genome with other flu virus strains, causing a much more dramatic mutation. It's even possible for a pathogen to spring from one host to another completely different host, for example from bird to human. The condition for this is that the human is infected simultaneously with an avian and a human flu virus strain. The genetic material from two different virus types could then conceivably be copied into a single cell, and during the assembly of this information for further distribution, the virus would then be able to create a mixture of the two. This is the scenario that has many virologists worried. If it were to happen, a virus could emerge which carries a combination of two, normally separate, and extremely dangerous characteristics: the deadliness of avian flu and the highly infectious nature of human flu. Such a virus could easily succeed in traversing borders and populations, resulting in the much-feared pandemic.