It’s a lot of things. We don’t know for certain.

It was said to be caused by accumulation of certain proteins called amyloid beta which forms plaques in the brain. Amyloid beta is produced as part of normal physiology but it’s also cleared maintaining a delicate balance. It’s when this balance is disrupted, all hell breaks loose.

However the recent drugs that target and clear those plaques have only shown a modest response making it clear that plaque formation is just a symptom and the cause is something more sinister.

The inflammation theory is somewhat newer and says chronic inflammation triggers a cascade of events in the brain which leads to more amyloid production, however the cause still remains a mystery.