The human being has evolved and continues to evolve in a word of microorganisms. This relationship is more a symbiosis than a competition and both take advantage of this "cohabitation". In fact, the microbiota represents a part of our body, from the skin to the gastrointestinal tract, and the total number of bacteria cells exceeds that of human cells. The microbiota is very important for the competition with the pathogenic microorganisms and for the immune system development and balance. Some viruses have integrated their genomes into the human genome. Some viruses, especially herpes ones, cause a persistent infection that requires control of the immune system throughout life. A small part of the microbes is, or can become, pathogenic; it depends on the microbes but, more often and much more, from the host himself. Epidemiological data about infant mortality in Europe, from the second half of the 19th century up to after the Second World War, clearly show how the impact of some pathogens on the infant mortality steadily decreased as a result of the improvement of living conditions. Epidemiological data about the second half of the 20th century clearly show an inverse correlation between the incidence of some common infectious diseases and the incidence of some immune disorders: is this a simple coincidence or some kind of causal relationship can be hypothesized?
Keywords: Vaccines, Vaccine policies, Law 119/2017, Bioethics, Science, Medical conscience.