Alzheimer's disease accounts for 50 to 60 percent of people with dementia and is the leading cause of senility (Psychology from inquiry to Understanding; p. 268). This is a disease that affects primarily older people, over the age of 65. As of now there is no evidence that suggests the precise cause of Alzheimer's disease. But there has been a correlation in patients with Alzheimer's disease and an abundance of senile plaques and neurofibrillary tangles. This correlation does not mean that this is what is the cause of Alzheimer's disease. A primary way to treat Alzheimer's is drugs that boost the amount of acetylcholine in the brain.
It is very tough seeing someone go through Alzheimer's. I know this because my Grandma currently has Alzheimer's disease. It is not an instantaneous event where one day they remember things then the next day they don't, but instead their memory slowly begins to fade starting with their most recent memories and then working back to their earlier memories. They also become distant because it is very hard for them to follow conversations.
There are multiple ways to reduce the risk of getting Alzheimer's. All of the ways to the risk of Alzheimer's are for the most part obvious and include, healthy diet, exercise, reducing tobacco and alcohol use and engaging in intellectual activities. Hopefully someday there will be a cure to Alzheimer's but for now all you have to do is live a healthy lifestyle, that shouldn't be too hard with all the gyms around nowadays.