The Virus Most of Us Carry for Life: Epstein-Barr, Mono, and What Your Genes Decide

If you have ever had mono — the two-weeks-of-sleeping, throat-on-fire illness that runs through dorms and high schools — you met Epstein-Barr virus. If you have never had mono, you have almost certainly met it too; you just did not notice. About 90-95% of people carry EBV, and once it arrives it never leaves. It parks itself in B cells, a kind of white blood cell, for life.

Which raises a question nobody could really answer until this year: if nearly everyone has the same virus, why does it stay quiet in most of us and not in some? A study in Nature found a way to ask 822,000 people at once — and part of the answer is in your genes. Four of the positions it found are already on this site.

Reading a virus out of a genome file

The clever part of Schmidt et al. 2026 is that they did not test anyone for the virus. When a blood sample is whole-genome sequenced, the machine reads every scrap of DNA in the tube — including any viral DNA that happens to be there. So the team searched the raw sequencing reads of 486,315 UK Biobank participants and 336,123 All of Us participants for pieces of the EBV genome. They found them in 16.2% and 21.8% of people respectively. That does not mean the rest were virus-free — nearly everyone carries it — it means those were the people with enough viral load in their blood cells to show up, which the team confirmed with separate measurements. Being EBV-read-positive went with HIV infection, immunosuppressive drugs and current smoking: things that loosen the immune system's grip.

The genes that hold it down

Then the genome-wide scan. The loudest signals were exactly where you would want them to be: in the MHC, the region that builds the molecules your immune cells use to show fragments of invaders to each other. The study found 54 independent HLA alleles, across both MHC class I and class II, associated with whether EBV showed up in the blood — plus 27 regions elsewhere in the genome, and an interaction between class I alleles and a gene called ERAP2 that trims the fragments HLA molecules display.

Four of those outside-the-MHC positions are on this site, from the same study: rs13098877 in LPP, rs884186 in KSR1, rs9828869 near ILDR1, and rs34557412 in TNFRSF13B — a gene for a receptor on B cells, the very cells EBV hides in, and a position this site also links to lymphocyte count. Each nudges the odds of a detectable virus a little. None of them decides it. That is the usual shape of these things, and it is worth saying plainly every time.

Why a virus this ordinary matters: the MS connection

Here is the reason anyone cares which genes control a virus almost everyone has. In 2022, a study in Science (Bjornevik et al.) followed more than ten million young adults on active duty in the US military, 955 of whom were diagnosed with multiple sclerosis during their service. The risk of MS rose 32-fold after EBV infection — and did not rise after other viruses, including cytomegalovirus, which spreads the same way. A blood marker of nerve damage, neurofilament light chain, climbed only after people converted to EBV-positive. The authors concluded that EBV is the leading cause of MS.

The 2026 study adds the genetic half of that story. People with MS carried a higher polygenic burden of the EBV-positive signal at MHC class I — driven by one allele, HLA-A*02:01 — and people with rheumatoid arthritis carried more of it at MHC class II. The same signal overlapped genetically with inflammatory bowel disease, hypothyroidism and type 1 diabetes. A virus that nearly everyone carries, a set of immune genes that decide how tightly it is held, and a list of autoimmune diseases at the other end — that is the shape of it.

So the next time someone mentions mono as a thing that happened to them once in college, the honest reply is: it is still there, in almost all of us, and your immune genes are quietly deciding, right now, how much of a say it gets. The infection and immunity topic collects what this site holds on that decision, and the multiple sclerosis page holds the variants on the other side of it.