Alzheimer's is a prion disease. Prions are proteins that cause other proteins to misfold. What's interesting is that most prion diseases are transmitted through ingestion or physical contact - possibly there's a correlation with population density and/or sanitation. It's also possible that there are low occurrences in ancient populations because most people died of other things. For example, 200 years ago bacteria diseases were the most common cause of death. Now that we have conquered most infection disease the primary cause of death now is cancer and cardiovascular issues.

https://www.nature.com/articles/s41591-023-02768-9