It may looks strange, but HIV is, in this regard, less efficient than flu.
Flu is airborne: it is perfect for a rapid spread in a world where ships could still carry large amounts of people fast enough from point A to point B and happily spread toaward unsuspecting targets. And once it gets a hold of its target, symptoms rapidly incapacitate it.
HIV requires direct blood exchange: in this regard is not worse than other venereal diseases. It will kill a lot of people, like those diseases did, but since it is slower in acting, once symptoms are visible on the body, medics and authorities can start tracking it down to the closest to patient zero. Triage and precautions can be refined and applied better than with flu, which is also a mutant and can hit again regardless of precautions -that is why we need to vaccinate, to keep it at bay from the weakest targets (sick people, elders, kids and babies)
So no, the answer is: HIV wouldn't change a thing unless all soldiers on all fronts had had sex with infected people, coincidentially in the right time frame so that at the end WWI they got sick with AIDS as well
In regard to the Fascism, once it has been determined that sexual contact is directly involved, it will cause authorities to push for stricter laws regarding monogamy and racial purity, but the whole legal corpus would be unaffected for what the general fascist philosphy is concerned.