Not quite what seems to be the issue. The theory seems to be a common virus (or several), which in most cases is harmless but in dudes with a certain genetic make up or susceptibility, leads to MS. At least that’s one theory anyway, If I am reading it correctly. Thus our OP has already been exposed to said common virus, and apparently is not susceptible. From what I have read, you’d have to be a bubble-boy to not be exposed at some time in your life.
Everyone has HLA genes.
Different ones make you more susceptible to different types of autoimmune and rheumatological diseases.
eg.
DR2 - sarcoidosis/lupus
A3 - primary haemachromatosis
B27 - ankylosing spondylitis, inflammatory bowel, Reiter’s, psoriatic arthritis, MS
B8 - Graves’ disease, coeliac
DR2 - MS, hay fever, Goodpasteur’s, SLE
DR3 - DM type 1, SLE, Sjögren’s
DR4 - DM type 1, RA
DR5 - Pernicious anemia, Hashimoto’s thyroiditis
etc.
Having said all that…
Autoimmune disease aren’t common and just having a certain HLA type doesn’t ensure you’ll develop any of the above conditions.
Only 50% of identical twins of people with MS develop it, and only 20% of non-identical twins develop MS. These are people who share genetic susceptibilty, early childhood exposure to viruses and often lifestyle and socio-economic similarities.
WarmNPrickly your friend is probably being treated with immunosuppressant medications to control the MS - they are more likely to pick up a nasty bug from your daughter, and for that reasons she shouldn’t be visiting him.
That chances of her “catching” MS are almost certainly non existent.
The chances he catches a nasty virus from a small (presumably snotty nosed and not toilet- trained) child…reasonably high.
My sister-in-law has severe chronic progressive MS – she’s permanently confined to a wheelchair and has no mobililty below her neck. We have absolutely no qualms about our 2- and 3-year old children playing, hugging, kissing her, etc. They also have four young cousins under 13 who are the same way (and their parent is a nurse practitioner).
None of her four brothers and sisters – who grew up with her – developed MS (they’ve all been tested). And they had daily contact with her for decades, and share the same gene pool. We’re not the slightest bit worried.
My sister works for the MS Society here in Northern Ireland, so we get a little bit of info dropped off now and again on the illness. There’s a line of thought that it’s related to a genetic line that works its way back to Scotland and those of Scottish decent are more prone to suffering it. The worst rates in the world are found in Scotland, followed by Northern Ireland, which in the past received a lot of immigration from Scotland.