One of the biggest risk factors for most types of dementia is age. Although some forms of dementia may affect those under 50 – some even occur in children – the risk of dementia occurrence increases rapidly as people age. For example, the number of people with Alzheimer's disease doubles every five years after the age of 65, according to the National Institute of Neurological Disorders and Stroke (NINDS). However, it is important to note that dementia is not part of the normal aging process. Some people have a long lifespan and experience only a very slight mental decline.

Another important risk factor for many forms of dementia is genetics and family history. Some forms of dementia can be traced to genetic abnormalities. This is the case with Huntington’s disease, which is passed from parent to child through a gene mutation. Additionally, people who have a sibling or parent with Alzheimer's disease are more likely to develop the disease themselves. This risk is even greater if more than one member of the family has been diagnosed with Alzheimer's disease. The specific relationship between genetics and dementia is still being studied.

Some causes of dementia include:

• Stroke. A stroke is a life-threatening event in which part of the brain is deprived of adequate oxygen (hypoxia). Also known as a cerebrovascular accident (CVA) or a “brain attack,” a stroke occurs when a blood vessel in the brain bursts or becomes clogged by a blood clot or other mass. This prevents oxygen and nutrients from traveling to nerve cells in the affected area of the brain. These nerve cells can die within minutes, and the area of the body they control can cease to function. In many cases, this damage is permanent, especially if the patient is not treated immediately. Stroke is the most common cause of vascular dementia.
  

  

• Brain injury. Dementia may occur following a traumatic head injury, depending on which part of the brain is affected. Repeated concussions (such as those sustained while boxing) increase the risk for dementia pugilistica, a form of dementia that can also result in Parkinson-like symptoms.
  
  
• Malnutrition or metabolic disorders. Malnutrition is defined as the deficiency in one or more vital nutrients. Many people are malnourished without realizing it, especially if they do not eat a balanced diet. Deficiency in any of the B vitamins may result in dementia. Some people are unable to absorb or metabolize certain vitamins or minerals, which may lead to dementia. In some cases, treatment for malnutrition or metabolic disorders may reverse the dementia.
  
  
• Infections. Some infections, especially those that cause high fever, can cause many of the symptoms of dementia. These are often resolved once the infection has been treated. Infections that directly affect the brain, such as human immunodeficiency virus (HIV) and meningitis, can result in more severe and possibly permanent brain damage and dementia. An untreated infection with the sexually transmitted disease syphilis can lead to tertiary syphilis, which can include symptoms of dementia.
  
  
• Medication reactions. Some medications can cause memory loss and other symptoms of dementia as a side effect. These may include medications used to treat depression, seizures or Parkinson’s disease. Additionally, some medications interact with other substances to produce symptoms similar to dementia. People who experience symptoms of dementia should inform their physician of all medications they are taking. They should also consult with their physician about any vitamin supplements, topical treatments or over-the-counter products they are taking.
  

  

• Drug and alcohol use. Abusing most illegal drugs may lead to symptoms of dementia, which can normally be resolved once the substances are removed from the body. Severe alcohol abuse, especially heavy drinking followed by sudden withdrawal, can lead to a permanent form of dementia called Wernicke-Korsakoff disorder.
  
  
• Poisoning. Some substances such as lead and mercury can cause dementia if people are exposed to unsafe levels. Mercury poisoning is generally associated with broken thermometers or working in a thermometer factory. Lead poisoning is more common and can occur after exposure to lead-based paint used in older homes, breathing contaminated air or eating contaminated food.
  
  
• Brain tumor or lesion. Tumors or brain lesions, which can cause pressure to build up in the brain, can disrupt the function of neurons. This can lead to symptoms of dementia. In certain cases, removal of the tumor or lesion can restore cognitive function, although sometimes the damage may be permanent.