Starting my research by referring to Wikipedia, I found out that this is usually the case with those afflicted with HS – they do not even tell the rest of their family, "because they are too ashamed!" The disease is now known to be an autosomal dominant disorder, in other words one which can be inherited from a parent who carries only a single copy of the gene. It is not infectious—a godsend because the suffering must be horrendous! Once thought to be a rare condition, probably because it was so rarely reported, it is now known that that is not the case and, for an unknown reason, women are several times more likely than men to develop it. The cause is unknown but, as far as I'm concerned, the fact that people can inherit it is bad enough.
Although the literature on the Internet seems to support the fact that the autosomal gene has not yet been mapped, I did find one reference in which the writer maintains that it has, but unfortunately provides no details. It would help if one could establish the gene pool that is at risk, but I've not been able to find such info for my distraught new acquaintance. Judging by her blonde colouring, I have even written to a good friend who is a doctor in Sweden, but he also could not find proof of any such details concerning this ghastly disease, which must be worse than leprosy.
Exploitation on YouTube
It was a huge mistake to consult YouTube at a friend’s suggestion—not because the articles and images posted there for the right reasons were so tragic that I wept, but mainly because, as is so often the case, it made me livid to see, among those postings, the usual exploitation of an illness for the purpose of marketing useless cures. Meanwhile there is little I can do but try to promote awareness!