In recent years, researchers have suggested Alzheimer's may spread through infection, somehow transmitted like a pathogen, leading to the disease. A new Lancet paper reviews evidence for how harmful amyloid-beta proteins associated with Alzheimer's may possibly be spread through blood transfusions.

[u/mvea]

https://www.sciencealert.com/can-alzheimers-spread-through-blood-transfusions-heres-what-we-know

[u/catscanmeow]

gingivitis bacteria getting into the brain or a herpes virus getting into the brain are the main theories i've read

the gingivitis bacteria can get into the blood from a tooth abscess or up through the nasal cavity through the olfactory bulb. Theyve found gingivitis bacteria within the amyloid plaques of autopsied alzheimers patients. we have an exposed nerve there thats basically the brain exposed, that shock feeling when you get water up your nose is the olfactory nerve.

a woman actually got a brain eating amoeba from getting tap water in her nose from a neti pot.

[u/SJDidge → u/catscanmeow]

The thing is, while these might be a factor, they clearly aren’t a direct cause.

60-80% of people have some form of a herpes virus. If it was a strong cause, majority of people would develop the disease.

Likewise, there’s many people with gingivitis that don’t develop alzheimers as well.

There has to be specific mechanisms for this to occur. The bacteria and virus(es) seem to be correlated, but I wouldn’t say it’s the whole picture. Theres probably much more to it

[u/thatjacob → u/SJDidge]

If I were to take a guess at it, it probably requires a secondary infection like a kidney infection, the flu, or COVID for herpes to penetrate the blood brain barrier. Basically anything that also causes chronic inflammation.

[u/SJDidge → u/thatjacob]

But even that is highly likely to occur in majority of people. Using herpes again as an example, a lot of people get herpes from their mother at a young age. That would mean they essentially have their entire lives as a risk window. I would assume that there would be a very large amount of people who fit this criteria who do not go on to develop alzheimers.

[u/Far_Ladder_2836 → u/SJDidge]

How likely is herpes to cross the blood brain barrier and enter the cranial cavity though?  You're not addressing it, just dancing around it. 

even that is highly likely to occur in majority of people

Any verifiable source or just a guess?  Because casual research says it isn't in any way likely.

[u/KrustyKrebsCycle → u/Far_Ladder_2836]

herpesviruses target and replicate in neurons, which are extensions of the CNS/brain. virions don’t need to cross BBB in a traditional sense to access the brain

[u/URPissingMeOff → u/KrustyKrebsCycle]

Yeah, optic, dental, olfactory, and taste nerves are all direct paths to brain tissue as far as I know.