Complex sentence: Dementia is not a single disease, but rather a non-specific illness syndrome (i.e., set of signs and symptoms) in which affected areas of cognition may be memory, attention, language, and problem solving.
Split into simpler sentences: Dementia is not a single disease, but a non-specific illness syndrome (i.e., set of signs and symptoms). Affected cognitive areas can be memory, attention, language, and problem solving.