The most common type of dementia is Alzheimer's disease, which is thought to affect 4.5 million people in the United States, according to the National Institutes of Health. Alzheimer's disease is a progressive, degenerative neurological disorder that occurs when neurons in the brain die or break their connections with other neurons. This is believed to occur because of abnormal lesions in the brains of people with Alzheimer's disease called beta-amyloid plaques and neurofibrillary tangles. These plaques and tangles disrupt brain function and are known to increase in quantity as Alzheimer's disease progresses.

Another common form of dementia is vascular dementia, which accounts for up to 20 percent of all dementias according to National Institute of Neurological Disorders and Stroke (NINDS). Vascular dementia is caused by lack of blood flow to the brain, usually following a stroke. It is the second most common type of dementia in older people. Unlike Alzheimer's disease, vascular dementia often develops suddenly after a cerebrovascular event and may or may not get worse over time. Some patients may recover certain aspects of cognitive function after a period of time.

There are different types of vascular dementias. Multi-infarct dementia (MID), for example, is caused by multiple small strokes in the brain. People with MID can experience only isolated symptoms of dementia such as memory loss or language impairments, depending on the area of the brain damaged by the strokes. A rare type of vascular dementia, Binswanger’s disease, is caused by damage to the small blood vessels in the brain. People with Binswanger’s disease often experience other symptoms such as urinary incontinence, clumsiness and problems walking.

Other types of dementia include:

• Lewy body dementia. This progressive dementia is similar to Alzheimer's disease in that its symptoms seem to be caused by the build up of pieces of protein (Lewy bodies) within the neurons of the brain. People with Lewy body dementia also experience pronounced lapses in concentration or alertness, visual hallucinations and Parkinson’s disease-like motor function problems. People with Lewy body dementia often have clinical signs of Parkinson’s disease and Alzheimer's disease. However, the true relationship between Lewy body dementia, Alzheimer's disease and Parkinson’s disease is not yet fully understood.
  
  
• Frontotemporal dementia. Also called frontal lobe dementia, frontotemporal dementia results from damage to the frontal and/or temporal lobes of the brain. People with frontotemporal dementia develop changes in a protein known as tau. Frontotemporal dementia is a type of dementia that results in mood and personality changes, which means that some people with frontotemporal dementia may exhibit inappropriate behavior (e.g. stealing, cursing) and have problems in social situations.
  
  
• Corticobasal degeneration. This progressive dementia is more common in people over age 60 and is the result of nerve cell loss throughout the brain. People with corticobasal degeneration experience symptoms that are similar to Parkinson’s disease along with the conventional symptoms of dementia, including, most notably, apraxia (the inability to make purposeful movements).
  
  
• Huntington’s disease. This type of dementia is hereditary and is caused by a genetic abnormality. Children whose parents have Huntington’s disease have a 50 percent chance of developing the disorder, according to NINDS. Unlike many forms of dementia, the symptoms of Huntington’s disease, which include personality changes, psychosis and abnormal muscle movements, begin to develop in the patient’s 30s or 40s. After diagnosis, people with Huntington’s disease live for about an average of approximately 15 years.
  
  
• Creutzfeldt-Jakob disease (CJD). Although CJD has received a large amount of media coverage over the last few years, the disease is still very rare. CJD is a type of progressive dementia caused by a prion, which is an infectious protein. Individual can be infected with the prion by consuming the brain or spinal tissue of an animal that has been infected. In animals, a variant of CJD is calledh mad cow disease. In can also be hereditary. People with CJD may experience personality changes, hallucinations, blindness and problems with coordination.
  
  
• Niemann Pick disease. Dementia may occur as a symptom of this group of genetic disorders. Patients with the disease cannot metabolize cholesterol and other types of fats normally, and these fats accumulate in the brain. The disease may begin during childhood, adolescence or early adulthood.