Aids would have behaved more or less exactly like syphillis did.
AIDS is a sexually transmitted disease, and we have the chance of
another STD which appeared out of nowhere around those times
(actually about a century later): syphilis.
When it was imported from the Americas, it quickly spread across Europe, > limited only by its own feature of disfiguring the persons it infected
(which is actually thought to have made it evolve less virulent, to
allow the victim to still be attractive enough to have intercourses).
Mind that religious objections to a multi-partner-oriented sexual life
didn't succeed in stopping the propagation.
(out of other answer)
Primärstadium, Lues I
Auch unbehandelt heilen die Geschwüre von selbst nach ca. 4–6 Wochen ab, weshalb die Erkrankung oft ignoriert oder nicht erkannt wird.
Sekundärstadium, Lues II
Acht bis neun Wochen nach der Ansteckung
Alle Hauterscheinungen (Syphilide) heilen nach ungefähr vier Monaten
ab, so dass manche Patienten von ihrer Infektion wenig bemerken.
Unbehandelt kommen sie innerhalb verschiedener Zeitabstände wieder.
(und heilen wieder ab...)
Bei vielen Erkrankten kann die Syphilis in der folgenden Latenzzeit zu
einem Stillstand kommen; die Erreger sind jedoch noch im Körper des
Betroffenen. So kann sich nach Monaten oder Jahren eine Spätsyphilis
entwickeln. Der Infizierte ist ansteckend, auch wenn diese Gefahr sinkt,
je länger der Patient beschwerdefrei bleibt.
Tertiärstadium, Lues III
Drei bis fünf Jahre später
The first stage of syphilis you often don't recognize as syphilis at all or ignore because the few ugly things on your skin disappear 4-6 weeks after infection.
The stage when you get ugly follows after 8-9 weeks after infection.
A few months after you got ugly, you get beautiful again.
A few months later, you get ugly again.
(and this repeats)
Syphilis may stop totally (or for just some months or years) at that stage, so you don't feel and look sick anymore but you are still infectious.
But, after 3-5 years after infection, the third stage of syphilis starts (thats when you get permanently ugly and may die)
So, like in the aids case, you have some people who look healthy although they are infected and spread the disease for years.
(yes, syphilis makes more of its victims permanently ugly in shorter time, but therefore it's more infectious (it can even be transmitted through food) (just think about medieval taverns)
so I think although there are differences
- faster in making you ugly (in average) (although you can stay beautiful for years and infect your whole town)
- much more infectious
- slower in making you ugly (in average) (although you can get ugly and die from HIV quickly, too + it's not as slow as you think when untreated-see HIV behavior when untreated -> > 90% die after 2-15 years, ~ 80% die after 5 years and 10 years.)
- much less infectious
The effects on a medieval world would be more or less the same.
(I think the differences compensate themselves)
So I would compare the actual syphilis epidemic with the black death epidemic and assume that Aids would behave the same as syphilis did.
Since syphilis stayed in europe until the first world war, (history lessons) I think syphilis was worse than the black death epidemic you're talking about (70.000.000-200.000.000 deaths). (caused more deaths since it had much more time)
=> Aids would be worse than the black death epidemic you're talking about as well.
(If anyone argues that aids would kill you faster in a medieval world because you get an immune deficit, that's true, but syphilis would kill you faster as well if you get additional diseases.