Dementia-Related Illnesses: Dementia is a term used to describe a general decline of more than one cognitive function. Dementia-related illnesses are a class of diseases that have such a generalized, deleterious effect on the brain that they usually result in dementia. Most of these diseases also have an incidious onset of symptoms and a progressive loss of intellectual skills. The most common dementia related illnesses are Alzheimer's disease and Multiple-infarction dementia. Some CNS infections also cause dementia, such as Jakob-Creutzfelt disease and viral encephalitis.