[An update on vaccines. Theory, realities and myths (I)].

Vaccines are biological products used to achieve active artificial immunization. Nowadays, vaccines are increasingly powerful, effective and safe. The list of vaccine-preventable diseases is expanding, which together with the increasing population likely to be vaccinated, it is expected that vaccine products will increase this century. This article is a brief but practical overview of the concept and types of vaccines, advances that have taken place recent decades on the fundamentals of the immune response and vaccine components (including the role of adjuvants). These concepts give rise to the criteria for when and how to vaccinate, to whom and why not to do it. With the hope that this text is useful, it ends with a brief overview as to why there are people who refuse vaccinations.