In Swedish dialects we find the terms ris, skäver and skärva referring to illnesses in children. The words are also parts of various compounds which refer to variants of the illnesses. The terms are linguistic expressions denoting two folk categories of illnesses, RIS and SKÄVER/SKÄRVA. These categories are investigated from a cognitive semantic perspective. The cognitive perspective argues that we organize our understanding of reality by using Idealized Cognitive Models (ICM) based on our physical, mental and emotional experiences of the world. The aim is twofold: to demonstrate the bases on which an experienced illness is placed in a certain category, and to show how a folk conception of illness is reflected in the word formation strategies. The word formation strategies emanate from notions of characteristic symptoms, and from notions of causes of illnesses. Both categories, RIS and SKÄVER/SKÄRVA, are based on a number of ICM's. The category RIS is a radial structure, which means that the category is held together although its members have no structural criteria in common. The category SKÄVER/SKÄRVA is a concentrating structure, meaning that all members share all structural criteria. There is a strong connection between word formation strategies and the structures of the categories. Terms related to symptoms refer to members of a category which are part of a radial structure, while terms related to causes refer to members of a category which are part of a concentrating structure. This can be explained by two of the basic assumptions of cognitive semantics: semantic content is structred and symbolized overtly on the surface form of a language and categories are conventional, based on cultural assumptions about the world.