This is a medical question of type : causes 
 What causes Alzheimer's Disease ? 
 There are two types of Alzheimers diseaseearly-onset and late-onset. Early-onset Alzheimers is a rare form of the disease that occurs in people age 30 to 60. It occurs in less than 5 percent of all people with Alzheimers. Almost all people with Alzheimers disease have late-onset Alzheimer's, which usually develops after age 60. Causes Not Fully Understood Scientists do not yet fully understand what causes Alzheimer's disease in most people. In early-onset Alzheimers, a genetic mutation is usually the cause. Late-onset Alzheimers arises from a complex series of brain changes that occur over decades. The causes probably include a mix of genetic, environmental, and lifestyle factors. These factors affect each person differently. Research shows that Alzheimers disease causes changes in the brain years and even decades before the first symptoms appear, so even people who seem free of the disease today may be at risk. Scientists are developing sophisticated tests to help identify who is most likely to develop symptoms of Alzheimers. Ultimately, they hope to prevent or delay dementia in these high-risk individuals. Risk Factors Some risk factors for Alzheimers, like age and genetics, cannot be controlled. Other factors that may play a role in the development of the diseasesuch as how much a person exercises or socializescan be changed. Lifestyle factors, such as diet and physical exercise, and long-term health conditions, like high blood pressure and diabetes, might also play a role in the risk of developing Alzheimers disease. For more information, see the chapter entitled Prevention. Older AgeThe Biggest Risk Factor Increasing age is the most important known risk factor for Alzheimer's disease. The number of people with the disease doubles every 5 years beyond age 65. Nearly half of people age 85 and older may have Alzheimers. These facts are significant because the number of older adults is growing. Genetics Genetics appears to play a part in both early- and late-onset Alzheimers disease. In early-onset Alzheimers, most cases are caused by specific genetic mutations permanent changes in genes that can be passed on from a parent to a child. This results in early-onset familial Alzheimers disease, or FAD. Most people with Alzheimers disease have late-onset Alzheimer's, in which symptoms appear in a persons mid-60s. No obvious family pattern is seen in most of these cases, but certain genetic factors appear to increase a persons risk. Many studies have linked the apolipoprotein E gene to late-onset Alzheimers. One form of this gene, APOE 4, increases a persons risk of getting the disease. But many people who get Alzheimers do not have the APOE 4 gene, and some people with the gene never get Alzheimers. Scientists have identified a number of other genes in addition to APOE 4 that may increase a persons risk for late-onset Alzheimers. Knowing about these genes can help researchers more effectively test possible treatments and prevention strategies in people who are at risk of developing Alzheimers -- ideally, before symptoms appear. Learn more about the genetics of Alzheimers disease.