Herpes, Gut Bacteria, and the Brain

A Roundup of New Neurodegeneration Research

This is just a quick roundup of several new studies that explore the role of viruses and bacteria in neurodegeneration in aging.

Viral causes of Alzheimer’s:
Tau tangles and amyloid beta are integral to the Alzheimer’s pathology, but just having amyloid-beta alone isn’t the whole story. Researchers have shown that amyloid beta plays a role in the brain's innate immune response to viral, fungal, and bacterial pathogens. It’s not just a toxic accident in the brain, but a way to trap viral particles.[ref]

A recent study from Harvard researchers takes this one step further and shows that one reason tau tangles form is to protect the brain from herpes simplex 1 (HSV1) virus. Tau acts specifically to fight viral infections, but in the process combines with amyloid-beta to form the plaque that causes Alzheimer’s.

From the study: “The combined antimicrobial activities of Aβ and phosphorylated tau resulting in Aβ plaques and neurofibrillary tangles, along with neuroinflammation, suggest that AD neuropathology may have evolved as an orchestrated innate immune host defense response to microbial infection in the brain.”[ref]

Your first thought may be: If Alzheimer’s is caused by a virus, can we just take an antiviral medicine to prevent it? You aren’t the only one thinking that…

A phase I clinical trial, published in JAMA in December 2025, tested this idea using the antiviral valacyclovir. The study involved 120 participants who were herpes simplex seropositive and who had early stages of Alzheimer’s. The results showed that taking valacyclovir, an anti-viral medication, was actually associated with a slightly worse outcome on the Alzheimer’s tests than the group receiving a placebo.[ref]

So it seems that valacyclovir is not the answer for someone who has Alzheimer’s.

Perhaps it is the timing? Treating herpes simplex with valacyclovir in elderly people who were in the beginning stages of Alzheimer’s may have been too late.

A large retrospective study in Taiwan looked at herpes simplex infection data and treatment data going back to the year 2000. The researchers compared data for over 8000 people with herpes simplex to an age and sex matched group of more than 25000 people without the virus, and then looked at who ended up with an Alzheimer’s diagnosis. They divided up the herpes simplex positive group into those who were treated with antivirals vs. those who weren’t treated.

The results showed that the untreated HSV-positive group was at a 2.5X increased risk of Alzheimer’s. However, the group that had taken anti-viral medications for herpes simplex was at a significantly reduced risk of Alzheimer’s. What’s unknown here from the insurance data and retrospective study is which specific anti-herpes medications were used in Taiwan to treat the virus.[ref] However, the implication is that treating herpes during the initial infection seems to greatly reduce Alzheimer’s risk.

If amyloid beta is part of the brain's immune defense against pathogens, then the next question is: how are those pathogens getting into the brain in the first place? A fascinating new study explains an unexpected route of entry.

Bacteria moving into the brain:

You may be wondering how pathogens get into the brain since we have a nice blood-brain barrier protecting the noggin.

A massive study this week showed that some bacteria take the gut-brain axis highway by hitching a ride on the vagus line.[ref]

Background context: A wave of research over the past decade has firmly established a connection between changes to the gut microbiome and changes to the brain. The gut microbiome shifts can be measured a decade before the onset of Parkinson’s and predict the disease. Even neurodevelopmental disorders, such as autism, often have measurable differences in the gut microbiome.

This new study involved mice fed a high-fat diet — specifically, it’s the high-fat, high-cholesterol diet that is used in studies to give mice atherosclerosis. It’s not what mice would normally eat. Shortly after the diet change, the gut microbiome composition shifted substantially, and intestinal barrier function was reduced.

The researchers checked to see if gut bacteria were escaping into the bloodstream and organs, but didn’t find that happening. Instead, they found that certain gut bacteria were moving up the vagus nerve into the brain.

The blood-brain barrier was still intact, and genome sequencing using DNA barcoding clearly showed that low levels of bacteria in the brain were the same bacteria as were in the gut.

Giving the mice antibiotics then caused big shifts in the composition of the gut microbiome. However, microbes still moved to the brain, just different ones than were seen in the initial study setup. Partially cutting the vagus nerve, though, decreased bacterial translocation to the brain significantly.

Taking it another step, the researchers used different mouse genetic strains that are used for modeling Alzheimer’s, autism, and Parkinson’s. Again, when fed the high-fat diet, they were able to show specific gut bacteria translocating to the brain through the vagus nerve.

The study was extensive and meticulous, as is needed for proving a novel finding. I would encourage you to read through it here (I’m not doing it justice with this explanation!).

Wrapping up:
I don’t have a lot of answers here, just musings on the very interesting studies. The blood-brain barrier blocks bloodborne pathogens and toxins, constantly protecting the brain. But it seems that pathogens are still sneaking in, and some may be using nerve pathways to get there. Recently making headlines are studies showing that the shingles vaccine may also decrease dementia risk. While I’m sure there are many variables involved there, including healthy user bias, it is interesting in that the shingles (chicken pox, varicella-zoster) virus can stick around for decades, remaining dormant in the nerve ganglion.

Finally, I’d like to circle back to a couple of previous articles. I recently wrote about the theory that motor neurons are the hard limit on lifespan. I’ve also written about the role of gut barrier integrity in preventing autodigestion. Both ideas come together here again in the role of the gut microbiome and gut barrier to protect against the translocation of viruses and bacteria into the brain.