As unavoidable mentioned, there are viral theories.
In Science, from July, "Can infections cause Alzheimer’s? A small community of researchers is determined to find out. Following up tantalizing links between pathogens and brain disease, new projects search for causal evidence", https://www.science.org/content/article/can-infections-cause...
In Science, from July, "Can infections cause Alzheimer’s? A small community of researchers is determined to find out. Following up tantalizing links between pathogens and brain disease, new projects search for causal evidence", https://www.science.org/content/article/can-infections-cause...