Still, the long tale behind this finding shows just how difficult this disease is and why progress toward preventing or curing it has been so slow.

Ever since Alzheimer’s disease was described by a German doctor, Alois Alzheimer, in 1906, there was only one way to know for sure that a person had it. A pathologist, examining the brain after death, would see microscopic black freckles, plaque, sticking to brain slices like barnacles. Without plaque, a person with memory loss did not have the disease.

There is no treatment yet to stop or slow the progress of Alzheimer’s. But every major drug company has new experimental drugs it hopes will work, particularly if they are started early. The questions though, are who should be getting the drugs and who really has Alzheimer’s or is developing it?

Even at the best medical centers, doctors often are wrong. Twenty percent of people with dementia — a loss of memory and intellectual functions — who received a diagnosis of Alzheimer’s, did not have it. There was no plaque when their brains were biopsied. Half with milder memory loss, thought to be on their way to Alzheimer’s, do not get the disease. And with such a high rate of misdiagnosis, some who are mistakenly told that they have Alzheimer’s are not treated for conditions, like depression or low levels of thyroid hormone or drug side effects and interactions, that are causing their memory problems.

Brain scans that showed plaque could help with some fundamental questions — who has or is getting Alzheimer’s, whether the disease ever stops or slows down on its own and even whether plaque is the main culprit causing brain cell death.