Introduction and terminology This chapter provides an introduction to the concept of syndromes and describes some genetically determ ined syndromes that are or may be associated w ith learning disability. Some basic genetic term s and concepts are discussed in Chapter 2. Terms that are used to describe how com m on diseases are include incidence and prevalence. The incidence of a disease is the rate of occurrence of new cases in a defined population over a given period of time. Prevalence is the proportion of a defined population that has a disease at a given point in time or over a given period of time.