It's called the Microbial / Infectious Hypothesis of Alzheimer's Disease

Dr. Oscar Fischer took one look at Patient #1’s teeth and immediately connected her severe gum disease to her brain-wasting condition. And unsurprising to he and his research team, her autopsy discovered oral bacteria in the brain. Excited with this novel finding, Fischer went on to perform dozens of autopsies to support his theory that oral bacteria was somehow getting into the brains of these patients with brain-wasting senility. 

Alzheimer, on the other hand, did very few autopsies. But he did make sure to publish his data and speak frequently on his theory, which gave rise to the disease being named Alzheimer’s. But because there were so few cases of this form of dementia back in the early 1900’s (our life expectancy was too short), interest in the topic waned – causing both researchers’ theories to loose traction. 

But as medical advances have extended our life expectancy, we’re seeing more of this brain wasting disease (it’s age-related first and foremost).  And with that, came a dusting off of both theories – Alzheimer’s and Fischer’s. 

Initially, because Alzheimer had published his data, his theory was the first that modern researchers grabbed onto. Now known as “The Amyloid Cascade Hypothesis” of Alzheimer’s – this theory rests upon the idea that proteins such as Amyloid and Tau are causing the brain-wasting disease.  But in the last 20 years, our new understanding of the brain supports that these supposedly harmful proteins are actually the brain’s immune response to foreign invaders – like microbes from the body (the mouth most often). This sudden break-through essentially resurrected Fischer’s Microbial/Infectious Hypothesis, causing it to gain ground over Alzheimer’s Amyloid Hypothesis to the point that AI now suggest that a vast majority of Alzheimer’s is actually caused by oral bacteria.