It is now known in Alzheimer's research that infection by certain bacteria and viruses may be associated with the development of Alzheimer's disease, and that these microorganisms may therefore be a risk factor.
The brain can also be infected: Viruses and microorganisms as possible causes of Alzheimer’s dementia
The American researcher Dr. Pat McGeer researched whether viruses could possibly cause the alteration of brain cells. He stained brain cells of patients who died with AD using a different staining method. Although he found no evidence of a virus, he found vast amounts of certain brain cells (so-called microglia). These cells only appear in such big amount under certain condition: inflammation! Dr. McGeer researched that microglia had already been discovered in the brain of dementia patients in 1919. However, this theory was not further investigated by that time, but is currently receiving new attention.