A rare disease is a disease that occurs infrequently or rarely in the general population. In order to be considered as rare, each specific disease cannot affect more than a limited number of people out of the whole population, defined in Chile as 0.18 in 10,000 citizens (Minister of Health draft of the law on rare diseases). This figure can also be expressed as 18 patients with rare diseases out of 1 million citizens. While 0.18 out of 10,000 seems very few, in a total population of 17 million citizens this could mean as many as 306 individuals for each rare disease. It is important to underline that the number of patients with rare disease varies considerably from disease to disease, and that most people represented by the statistics in this field suffer from even rarer diseases, affecting only one in 100,000 people or less. Some rare diseases do only affect a couple of dozens patients. These very rare diseases make patients and their families particularly isolated and vulnerable.