Published on November 3, 2025 6:30 AM GMT

Tl;dr:

Herpes viruses scare me (in a way other STDs don’t). I think there are some mechanistic and a-priori reasons to worry that HSV 1 or 2, especially orally, could have bad consequences later in life. In particular:

• Multiple other species of herpes can cause surprising long-term consequences (that sometimes go undiscovered for a long time)
• HSV 1&2 permanently live in your nerve cells
• Oral herpes can make its way into your brain

There might be cheap actions people can take to reduce their risk of contracting oral herpes (don’t do things like share straws or forks, don’t make out with a ton of people, ask short-term partners with HSV to take L-lysine). It’s a little confusing/unclear if it’s worth doing anything about this. I’ll try to make a follow-up post with suggestions.

The case for oral herpes being bad

Herpes viruses often have hard-to-predict latent bad effects

There are eight species of herpes virus that can affect humans. When you get infected with a herpes virus, there’s usually some initial sickness (chickenpox, mono, roseola). The initial sickness isn’t fun, but it’s not a huge deal. Your immune system kicks in, fights the cells harboring the virus, and you get better.

For many diseases, that would be the end of the story. But with herpes, it simply goes dormant. Remember, viruses are just genetic code, not cells unto themselves. Herpes infects cells that last decades or lifetimes (memory B and T cells and neurons). After an initial infection, it just… waits. Your immune system can only hunt down cells that are expressing suspicious viral proteins, it can’t read the DNA of every cell to vet it for herpes. So there’s no way to root out the remaining infected cells. These cells occasionally try to activate, and your immune system tries to suppress them and keep the herpes in check,^[1] but that’s the best you can do. There’s no cure, you’re stuck with this for life.

It turns out, having a herpes virus in your cells constantly is… not great. But how bad is it and what exactly are the consequences?

Some types of herpes directly hurt your cells

For HSV 1&2, the herpes virus can occasionally break out of your nerve cells and infect your skin. This causes the infamous mouth and genital cold sores people commonly associate with the word “herpes”. It can be uncomfortable but it isn’t a very big deal. Your immune system gets on top of things and clears it up in a week or two.

Chicken pox (VZV) is a similar story, the virus jumps from your nerves to your skin, causing shingles. But when this type of herpes reactivates, it can also inflame the nerve cells it lives in, which leads to permanent nerve damage 10-18% of the time.

Some types of herpes cause your immune system cells to divide too much, increasing cancer risk

EBV and HHV-8 infects B cells, then produce proteins that encourage these cells to reproduce so that there can be more infected cells in your body. These proteins can cause cancer. (Your immune system can mostly keep this in check, so it’s not actually a huge issue unless you’re immunocompromised.)

Of course, a lot of stuff causes cancer, and your risk is probably higher from sunshine than from EBV. But I think it’s notable that unlike most carcinogens, the cancers herpes causes are herpes-specific and don’t tend to be caused by other things.

This isn’t as relevant to concerns about oral herpes, since that infects neurons and neurons don’t replicate. But I do think it’s a sign that there’s all kinds of weird fuckery that might go on from having a herpes virus in your cells.

Having your immune system constantly fighting a battle can make it overreactive

If you have mono (EBV), your immune system gets really, really good at fighting the herpes proteins. Over the course of decades, they get so good at it that they occasionally start attacking other proteins that look a little bit like the herpes proteins. Unfortunately, the myelin sheaths that cover your neurons look a little bit like herpes, so you can wind up with MS (~0.1% chance given you have EBV). Or so the leading theory goes, the fact that EBV causes MS was only confirmed in 2022 and the mechanism isn’t fully understood. Many things about herpes in general aren’t that well understood.

I doubt that any of the exact things that make other types of herpes so pernicious later in life will be true for oral herpes. But they do illustrate why it seems heuristically bad for your immune system to be suppressing a viral infection all the time. There are lots of different things that can go wrong, and in many cases they can be tricky to identify and confirm.

Still, most of these effects later in life are either rare or not that bad. But oral herpes in particular gives me pause because…

Oral herpes can infect your brain

One study tried autopsying about 584 people who were about as representative as you can get of the general population whilst being a corpse, and found 2% of them had HSV 1 or 2 in their brain. (~86% of them tested positive for being infected with HSV 1 or 2 at all.) Another study (which has a smaller sample size and is older, so I trust it a little less) puts it at 35%. I’m as frustrated as you are that the numbers are more than an order of magnitude different, but both of them seem too high for my comfort.

I guess it travels to the brain through the trigeminal ganglia (it seems to often infect this nerve and this nerve goes to your brain). So I’m only worried about oral herpes, not genital herpes, because, as dumb as it sounds, your mouth is closer to your brain and your brain seems really important.

Oral herpes is linked to Alzheimer’s

Here’s a review paper titled “Overwhelming Evidence for a Major Role for Herpes Simplex Virus Type 1 (HSV1) in Alzheimer’s Disease (AD); Underwhelming Evidence against”. The paper is in a mid-tier journal and does not represent scientific consensus. But it does a fine job laying out the general case and shows this is something the scientific community generally takes seriously.

The two best pieces of evidence in favor of this theory (summarizing the paper):

• A couple large studies show people with HSV infections who were treated with antivirals had dramatically reduced dementia risk. One Taiwanese study of 33,448 people showed antiviral treatment reduced dementia risk by ~90%.
• The APOE-ε4 gene increases your risk of cold sores and your risk of Alzheimer’s (source).

Quick sanity checks aren’t enough to rule out notable cognitive consequences

I’ve tried to run a bunch of sanity checks to rule out major effects. For example I looked at the IQ of people with and without HSV. One problem is there’s not that many studies. But the bigger problem is the studies all show people with HSV have lower IQ (between 3 and 15 points, depending on the study). They tried to control for some obvious confounders, but this whole thing is so terribly, terribly confounded.  Probably the results of the study are just confounders. But so far, I have not found any sanity check that has quelled my fears. (Though note that given that half the population has herpes and half doesn’t, it can’t actually make a 15 IQ point difference, otherwise it would explain far too much of the variance in IQ that’s already been explained by other factors.)

Don’t panic

I don’t want to fear-monger.

First, I think it’s not worth worrying about any kinds of herpes other than HSV 1&2 unless you are a child. HSV 1&2 are contagious after the initial infection stage (unless you like licking shingles blisters).

During the initial infection stage, mono and rosella (and chicken pox, before we had a vaccine for it) are very contagious, so much so that they’re commonly contracted in childhood. That means (1) you’ve probably already contracted these (95% of people have had mono), and there’s nothing more to be done, and (2) if you don’t spend time around children, you’re probably protected by herd immunity.

(If you have a child, I think it’s not crazy to try and stop them from ever getting mono. But I haven’t thought much about this.)

I also think genital herpes is more-or-less fine (because your nether regions are far away from your brain), except for the fact that it can spread to your mouth and become oral herpes.

Oral herpes can only be spread by direct oral contact with an infected individual.

Of course, you might make a lot of oral contact with infected individuals; ~half of America has oral herpes. For at least 70% of people, they never experience noticeable symptoms.

The fact that half the planet has oral herpes bounds how bad it can be. If it was extremely terribly awful, the effect size would be so large that someone would have noticed.

There’s also lots of things out there that can kill you or cause cancer. There’s sunshine and sweets and grilled food and bonfires and plastic and who knows what else. In general, I think it’s healthy to be pretty laissez-faire and don’t freak out about every health association you see.

I’ll probably do a follow-up post with some recommendations, but I do not think anybody should lose a bunch of sleep over this (after all, losing sleep might also be bad for your brain…).

1. ^{^}

   You might wonder if the virus does occasionally express itself, why your immune system can’t root it out then. One issue is only a small number of infected cells will ever actually express themselves. Another issue is nerve cells never regenerate, so your immune system won’t kill them even if they’re infected with herpes, it will just try to suppress their expression of herpes (it’s cool that your immune system can do that and doesn’t randomly kill your neurons!).