In epidemiology or in epizootiology, there are some infectious diseases that have potential for significant reduction of the susceptible species population. Over the past few decades, epidemiologists were concentrated on diseases that were 'modern' and made front-page news in tabloids. One should recall diseases like bovine spongiform encephalopathy, SARS and AIDS syndromes. However, we should always be aware of the most dangerous diseases such as our old friend, influenza, or simply, flu. In the past decade, we heard about 'bird' or 'swine' influenza. It is the same disease for different animal species as well as for man. Influenza owes its characteristics to specific virus biology as well as to the epidemiology-epizootiology characteristics of the susceptible species. Antigenic changes that took place thanks to reassortment mechanisms of the viral gene segments cause the onset of the new antigenic combinations of the hemaglutinin and neuraminidase molecules. As a result, new H and/or N antigenic formulas appear for the first time in totally susceptible animal and human populations. That means that in such circumstances, no person in the world is immune to the virus. In that case, such a virus can cause a pandemic with disastrous consequences since influenza is a disease with significant mortality, especially in some segments of the human (as well as animal) population. Birds and swine are virus reservoirs, but these species are at the same time live test tubes in which the virus resides, changes and adapts itself not only to the original species but to other species as well. That means that there is no 'bird' or 'swine' flu. Influenza is an infection of several important animal species as well as man that have potential not only for the reduction of the population size but, in case of the human population, for influencing social and economic life. .