Causative subtypes of dementia may be based on a known potential cause such as Parkinson's disease, for Parkinson's disease dementia; Huntington's disease for Huntingtons disease dementia; vascular disease for vascular dementia; brain injury including stroke often results in vascular dementia; or many other medical conditions including HIV infection for HIV dementia; and prion diseases. Subtypes may be based on various symptoms as may be due to a neurodegenerative disorder such as Alzheimer's disease; frontotemporal lobar degeneration for frontotemporal dementia; or Lewy body disease for dementia with Lewy bodies. More than one type of dementia, known as "mixed dementia", may exist together. Diagnosis is usually based on history of the illness and cognitive testing with imaging. Blood tests may be taken to rule out other possible causes that may be reversible such as an underactive thyroid, and to determine the subtype. The Mini-Mental State Examination is one commonly used cognitive test. The greatest risk factor for developing dementia is aging, however dementia is not a normal part of aging. Several risk factors for dementia are described with some such as smoking, and obesity being preventable by lifestyle changes. Screening the general population for the disorder is not recommended.