The different categories, the so-called “International Classification of Diseases” (ICD), are defined by the World Health Organization. As shown in the table, the diseases vary in several points. These are the age of onset, social interaction, communication and behaviour. An individual belongs to a certain category, depending on the areas of impairment. All three may seem similar in symptoms, yet are not the same. Still, people within one group show similar symptoms. The groups can be used to search for an underlying cause of the disease and to provide a better treatment.