This illustration shows how Alzheimer's Disease affects the brain. There is a distinct difference between the healthy brain and the mildly affected one, not to mention the difference between the healthy and severely affected one. This picture shows that the cortical shrinks and the hippocampus shrinks while the ventricles become enlarged.
Although the cause of Alzheimer's is still unknown and therefor there no cures, it is known that genetics and age play a very large role in this disease. Getting very scientific, it is found that there is a correlation between those with three copies of chromosome 21 and Alzheimer's patients. However, it is still being researched if it is a major factor or not. Also, there is a gene on chromosome 19 that makes protein called apolipoprotein E4 is especially common in people who have Alzheimer's Disease. However it too has not been directly pointed out as a cause of the disease.
Researchers are still working toward finding a cure and are continually trying new treatments for patients of the disease.