When combined with results of other neurological tests, and in the context of a thorough medical history, atrophy of the brain (shown here in an MRI scan) sometimes indicates Alzheimer's. Simon Fraser/Science Source hide caption

itoggle caption Simon Fraser/Science Source

Laury Sacks and her husband, Eric. The actress and writer developed frontotemporal dementia in her late 40s and died in 2008 at age 52. Courtesy of Eric Sacks hide caption

itoggle caption Courtesy of Eric Sacks

Prion protein can be infectious, spreading from cell to cell in the brain. Here four nerve cells in a mouse illustrate how infectious prion protein moves within cells along neurites — wire-like connections the nerve cells use for communicating with adjacent cells. Science Source hide caption

itoggle caption Science Source

Greg O'Brien (left), with Colleen, Mary Catherine, Conor, and Brendan O'Brien, has been grappling with Alzheimer's disease for the last five years. Courtesy Greg O'Brien hide caption

itoggle caption Courtesy Greg O'Brien

Marian Grunwald (from left), Earl Elfstrom and Verna Matheson bounced a balloon back and forth with nursing assistant Rick Pavlisich on Dec. 13, 2013, at an Ecumen nursing home in Chisago City, Minn. Glen Stubbe/Star Tribune, Minneapolis St. Paul hide caption

itoggle caption Glen Stubbe/Star Tribune, Minneapolis St. Paul

A tangle of protein (green) in this scanning electron micrograph of a brain cell of an Alzheimer's patient lies within the cytoplasm (blue) of the cell. The tangle consists of clumps of a toxic form of tau. Thomas J. Deerinck/Corbis hide caption

itoggle caption Thomas J. Deerinck/Corbis

Doctors may eventually be able to diagnose "preclinical" Alzheimer's in patients who have abnormal brain scans but who aren't yet showing behavioral symptoms of the disease. iStockphoto hide caption

itoggle caption iStockphoto

University of Florida researcher Jennifer Stamps administers the peanut butter sniff test to a volunteer. Jesse S. Jones/University of Florida hide caption

itoggle caption Jesse S. Jones/University of Florida

The underlying biology of age-related memory glitches — in old mice and old people — is different from what happens with Alzheimer's, recent research suggests. Anthony Bradshaw/iStockphoto.com hide caption

itoggle caption Anthony Bradshaw/iStockphoto.com