The Great Plague is known as one of the worst pandemics in human history as it killed between 75 and 200 million people from 1347 to 1351 (according to wikipedia).
I would like to know if AIDS had erupted during the same period of time, would it have been more or less as deadly?
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.
In the case of AIDS we would have then sexual transmission and lack of countermeasure (silk or gut condoms used in those times are ineffective against the virus), helping the disease to spread.
Also don't forget that the common remedy for any illness was to slit open a blood vessel (usually near the elbow) and drain about half a liter of blood. The instrument used for cutting was not cleaned much and definitely not sanitized between patients. The same applies to tooth pulling, amputations and all the merry services of barber-surgeons. When you think about it, a single traveling barber-surgeon could have wiped out entire areas in a few years time. (credit at YElm for the hint)
The slow onset of the disease would have been also favoring its propagation, and the constant usage of the immune system in those times would have lead to quick deaths as soon as the deficit would have arise. Imagine an infected soldier in the latent phase, visiting various brothels across Europe before being killed by a small infection following a war wound.
Finally, considering that it doesn't show any major symptom (like fever), it would have not even raised any attempt to stop it (like isolating the sick). All in all, it would have been more deadly than the black plague.
The worst epidemic of AIDS we know of or have known of is the South African plague, which has as its highest infection rate only an excess of 15% (source). Assuming 100% lethality, that's only a 15%+ infection rate in small parts of Africa. The Black Death is estimated to have killed 30–60% of Europe's total population.
There is no evidence at all that AIDS can spread as quickly as the Bubonic Plague. On the contrary, the evidence demonstrates that it spreads much, much slower.
AIDS is likely more lethal than the Black Death, but it's only transmitted sexually and it has a much longer incubation time. 10 years compared to the Bubonic Plague's 2-6 DAYS. The average lifespan in 1300 was 31 years. This means the plague could affect ANYBODY during their 31 year lifespan but could only affect an individual for about 5 years assuming they became sexually active at age 15. (This assumes they weren't born with the disease.)
To achieve the kill rate of the Plague, the average person would need to be sleeping around so much that you could easily connect the average person to any other average person through sexual activity in the same way we play Six Degrees to Kevin Bacon.
While I have little doubt the average person is permissive in their behavior, I have a substantial problem believing the average person is that permissive. Far more common and easier to contract sexual diseases existed in the Medieval ages and yet had no where near the infection rate necessary to judge AIDS more deadly to the population of that time.
In the end, the issue is not the lethality of the disease or how hard it is to detect, but the ease with which it can be contracted. The Plague was the proverbial million times easier to contrat than AIDS ever was or is. Conclusion: the plague was worse... a lot worse...
Bubonic plague
Bubonic plague is an infection of the lymphatic system, usually resulting from the bite of an infected flea, Xenopsylla cheopis (the rat flea). So it can carry on after an area is completely dead, and, more importantly, it can occur without any infected people around so long as there are rats carrying the infected fleas.
Medieval Europe was a very messy place, the perfect breeding ground for rats, and the fleas went with them.
The infection takes three–five days to incubate in people before they fall ill, and another three–five days before, in 80 per cent of the cases, the victims die. Thus, from the introduction of plague contagion among rats in a human community it takes, on average, twenty-three days before the first person dies.
HIV/AIDS
You can get or transmit HIV only through specific activities. Most commonly, people get or transmit HIV through sexual behaviors and needle or syringe use.
Only certain body fluids—blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids, and breast milk—from a person who has HIV can transmit HIV. These fluids must come in contact with a mucous membrane or damaged tissue or be directly injected into the bloodstream (from a needle or syringe) for transmission to occur. Mucous membranes are found inside the rectum, vagina, penis, and mouth.
This means that you basically have to be very friendly with someone that has HIV or use syringes that have not been cleaned correctly after use by someone with HIV.
Left without treatment, the majority of people infected with HIV will develop signs of HIV-related illness within 5–10 years, although this can be shorter. The time between acquiring HIV and an AIDS diagnosis is usually between 10–15 years, but sometimes longer.
Spread
In the medieval times there were no syringes as we know them today, and the deeply religious time restricted widespread... "extra curricular activities" so HIV would not have been able to spread even as fast as it has in modern times, let alone medieval.
Lethality
While the black death is far, far more rapid, it did manage to burn itself out, however HIV/AIDS would have taken a lot longer to be noticed and for people to die from it, but would most likely still have burned out due to the discretionary customs of the time.
AIDS would not have been great. The plague was worse. Much worse.
There are people who simply don't get HIV https://en.wikipedia.org/wiki/Innate_resistance_to_HIV
And people of European descent have the highest occurrence. https://www.wired.com/2005/01/genetic-hiv-resistance-deciphered/
It's theorized by some that the survivors of the bubonic plague passed along a resistance that also helps against HIV.
Also, the really destructive effects that make a huge pandemic worse are the ripple effects of the sickness. If a substantial percentage of your working adults all get sick and die within a few weeks of each other - and parts of Europe supposedly suffered 70% death rates - society falls apart. Crops don't get tended. Livestock break loose or starve. Taxes aren't paid, or collected. Towns are abandoned. Even places without plague outbreaks suffer from starvation or secondary disease outbreaks. People get scared, trade fails. It's bad.
Even if people were dying of AIDs, it would be gradual. Society would survive fine. Of course, it wouldn't burn itself out either... so if you mean in the fullness of time, over centuries - then yeah, AIDS would end up killing more, but it would have less of an impact anyways. Medieval people dealt with mortality all the time, it was the suddenness of the plague wiping out so many at once that was such a debilitating thing.
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
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Auch unbehandelt heilen die Geschwüre von selbst nach ca. 4–6 Wochen ab, weshalb die Erkrankung oft ignoriert oder nicht erkannt wird.
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Sekundärstadium, Lues II
Acht bis neun Wochen nach der Ansteckung
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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.
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Tertiärstadium, Lues III
Drei bis fünf Jahre später
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(wikipedia)
In english: 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
( Syphilis - faster in making you ugly (in average) (although you can stay beautiful for years and infect your whole town) - much more infectious
vs.
HIV - 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.
As others have mentioned, plague is way worst simply because it can spread quicker, has a much smaller incubation period of 1 to 7 days compared to HIV's incubation period of several years and is much more lethal in the short term.
This isn't even counting Pneumonic Plague, which would spread even faster through person to person since it's airborne and is 100% lethal in a day or two (even today bubonic plague can still be lethal even with treatment).
