In order to understand why patients with Alzheimer's disease lose their memory, it's helpful to know how the brain creates memories. Once a person with a healthy-functioning brain learns something new or has an experience – whether it's when they're five or 55 – it registers in the part of the brain called the hippocampus, which sends that memory or info to other parts of the brain for storage. That "storage bank" retrieves the memory. When Alzheimer's starts to take hold, the hippocampus is one of the first areas of the brain to be damaged. Because this part of the brain doesn't remember that the event (or person) ever took place, it can't assist in retrieving the memory.
As the disease progresses, memories start to slowly dissipate. "The first thing that gets affected is the ability to take in new memories,
New memories that don't have the same emotional attachment are stored in a different place than other memories, which is why an Alzheimer's patient whose disease has progressed might remember an event from 20 years ago, but can't remember what they did 20 minutes ago.
As a result, you cannot teach a love one to not repeat themselves because the hippocampus cannot register the teaching or send it to the storage units of the brain -- this is caused by damage to the hippocampus.
When plaque builds up in the brain's nerve cells – one of the causes of Alzheimer's – memory and thinking are impacted. That is one of the causes of confusion among people with Alzheimer's disease and their inability to organise their thoughts or remember the faces of people closest to them.
This is a good link
http://www.alzheimers.org.uk/site/scripts/documents_info.php?documentID=114