How common is it for somebody to get alzheimer's?

It's common: 1 in 8 (13%) americans over age 65 have alzheimer's. That's 5.4 million people. 5.2 million of them are over 65. Here's a link to a pdf with lots of statistics and info on it: http://www.Alz.Org/downloads/facts_figures_2011.Pdf. Taking good care of yourself is the best way to keep from getting dementia - eat right, exercise, maintain a healthy weight, stay connected with loved ones.

Depends: The incidence of alzheimer's disease depends on a person's age and to some extent family history. 10% of people over 65, and >45% of people over 85 have ad. If there is late onset ad in the family (onset >65) the risk to children is only a few percent. If there is early onset ad (onset < 60) the risk can be up to 50%. The early onset type of ad is much, much less common overall.

Age related: Incidence rises with age-20% in persons over 80.