Nobody is really quite sure what exactly causes alzheimer's, but it can be from the build up of protein in the brain. The build up could be from either plaque or tangles that are inside or in the spaces of the nerve cells. One theory from scientists think that the plaque and tangles block nerve cells' ability to communicate with each other, making it difficult for the cells to survive. There is no actual cure for alzheimer's disease but there are medicines that can help and lots and lots of therapy. There are a few ways to prevent having alzheimer's disease, such as; staying active,taking vitamins,control your cholesterol and eating healthy. But these are not 100% full preventions, you can still get it. I can relate to alzheimer's pretty well since 3 out of 4 of my grandparents either have it or did have it. It's so hard showing up to their house and having them not even recognize you or forgetting everything that has happened. But you just have to learn to get use to it and help them remember things, as hard as that is.
I put this image on here from the movie "The Notebook" because in the movie, the woman has alzheimer's disease so her husband reads her a book of their life hoping that one day she will remember.