Since the beginning of human existence on planet Earth there has been one virus that has managed to pass through centuries and affect people all around the globe. It is known on all 5 continents as the the flu virus and, with the passing of time more and more information has been acquired giving scientists the possibility to create vaccines and medication.

Historical data shows the outbreak of three major flu epidemics in the last century in different parts of the planet: Spain, Asia and Hong Kong. An epidemic affects a single region, people or country without the possibility of outside spread of the disease. On the other hand, a pandemic-the major concern of modern society-has the ability to afflict millions of people in different parts of the world. So, the questions that come to everyone's mind are: Can there be a modern pandemic? What will induce it?

Flu is caused by three viruses: A, B and C. The former is highly contagious and has the capacity to infect humans, birds and animals, while viruses B and C affect only people. The influenza A virus has been the starter point of flu epidemics in the past, managing to cause economical crisis on Earth. With the passing of time virus A has become more powerful by developing new strains. The latest, the H5N1 virus which causes bird flu has been a major source of rapid human loss of life due to respiratory diseases. This subtype is contagious and can be transmitted between different species of birds or passed on to humans. For this reason, scientists worry the avian influenza may be the next pandemic to hit our planet.

Firstly, the H5N1 virus has a very complex genetic structure that changes constantly. As the virus comes in contact with very different organisms it is able to gain varied information, thus the new strains will make almost impossible the development of a vaccine. Even more, the human body does not have the capacity to create antibodies for every branch of the virus. With very little immunity it is almost certain people won't win the battle against this invisible and lethal enemy.

Furthermore, not only is this virus replicating but it is mixing with other viruses, it is acquiring and exchanging genetic material and may eventually be able to create such an evolved strain which will devastate continent after continent.

On the other hand, our health -already hanging by a string- is also threatened by the other strains of influenza A viruses which could provoke a world wide pandemic.

In the end there is no telling whether we will disappear by bird flu or our existence will continue untouched but, until now, chances are not on our side.