Diseases classed as rare or orphan are extremely important world-wide this century. They are defined in terms of their chronicity, the difficulty implied in the treatment of symptoms and the low prevalence in the country. In most cases, an orphan disease causes intellectual deficits in patients. The principal aim of this paper is to construct a literature review on the definition of rare diseases, as well as the definition of intellectual deficit. Both of these concepts are analyzed in terms of their national and international importance. To understand the importance of the study of rare diseases, three such diseases associated with intellectual deficit were selected to be researched at length. The diseases chosen were: Rett syndrome, Prader-Willi syndrome and X Fragile syndrome. These diseases are explained in terms of their diagnosis, intervention, prevention, support groups and types of evaluation more commonly used in the Colombian context