The Black Death (or, Great Death) was a pandemic that struck Asia and Europe from 1348 to 1350 and killed between 1/3 and 1/2 of Europe.
Pictured: The collective scream of "What the fuck!?" from every single Medieval citizen.
The epidemic that would become the Black Death began in China and was probably carried to Europe in shipments of silk and oriental spices (like you thought roofied toys were the worst things you could get from China). Rats hiding in the traders' food carried infected fleas, which were more than happy to jump on to anyone and anything that might be around. Luckily, rat fleas prefer rats, and if there are rats around they generally refuse to leap onto human beings. Unluckily, plague kills every rat it touches, and when the rats are dead, fleas just don't give a fuck anymore. Within a few months, every town and trading post along the Silk Road was positively littered with corpses.
Eventually, the disease made it to the beautiful city of Kaffa, Gateway to the Eastern and Western ports, city of destiny; you can see where this is going. The Genoese in Kaffa got the plague thanks to a trade with the Mongolians. Of course, what they were trading were "Corpses" and the chosen method of delivery was "Catapult."
You see, Khan Janibeg was a Mongolian leader with a history of perfectly reasonable responses to small events. For example, the siege of Kaffa started when a Muslim trader was killed in an argument in the market. The Khan's siege ended when his troops were struck down apparently by the 'hand of God' and, more exactly, the plague. While anyone a little more sane might have just packed up and gone home, Janibeg stayed there for two more months, periodically throwing plague infected corpses over the city walls.
While the plague burned through the city most of Kaffa's well to do decided the best decision was to flee. This would have been an even better decision if they hadn't been climbing into ships that were positively filled with rats. By the time the Kaffan fleet reached Genoa, anyone alive on those ships was enjoying blood foam at the mouth and bloody leakage at the ass. Unfortunately for Genoa, by the time any of them realized this, they'd traded fleas with the corpses, and within a few weeks the Genoese were making the same mistake the Kaffans had. Their fleet of seven ships would proceed to sail along the coast, infecting every single city they came near for the next few years.
French attempts to surrender to the Plague were met with mixed results
The plague infected all of Europe and while many areas to the north and east reported fewer-than-normal casualties (the normal casualty rate in Italian chronicles being "Everyone") only one area, Medieval Poland, is thought to have been spared completely, thus using up every last bit of luck the country would ever possess ever.
While most scientists now know that the Black Death was caused by the disease Yersinia Pestis or, colloquially, Plague and was spread by Reptoids and Deros from Zeta Reticuli, doctors at the time had a number of possible causes.
The thing in the middle is Earth, thus invalidating all potentially useful information in astrology ever.
The education of the average medieval doctor included such things as rhetoric (debate), anatomy (learned on pigs), and astrology. Yes. Astrology. Before giving the physical examination, most doctors would cast a horoscope for the patient, and there are court cases from the fourteenth century of people losing malpractice suits against doctors after injuring themselves under the wrong star sign. The Black Death was blamed by many doctors on certain unportentious signs (read: there was a comet) and the cure could be anything from prayer to sleeping where the stars couldn't see them.
Well, this one warrants some explanation. Because the disease killed so quickly, many people believed that the cause of death was, in fact, the spirit of the recently dead escaping through the corpse's eyeballs and punching any healthy people nearby who may have happened to look. Given that the plague spreads through breath this explanation isn't completely retarded, if you ignore the fact that they thought the soul of the person was physically beating them. So... just ignore that bit.
At the forefront of medieval hygiene, peasants holding flowers.
The most reasonable explanation for the Black Death (in that it was neither completely retarded and the cure actually worked) was the explanation that miasmas, or free floating bad air, were being inhaled by people, sickening and eventually killing them. The most common solution was to hold a very strong, sweet smelling herb or flower up to someone's nose, this was completely ineffective, of course. The effective solution was to light huge bonfires on the corners of streets and inside homes. These fires drove out fleas, which actually did stop anyone wealthy enough to be able to keep up a huge fire at all times (which was, mostly, the Pope).
Guys whipping themselves to appease God or whatever.
The typical fall-back for religious leaders and crazy guys who owned whips, the blame God approach usually ended with fasting, pilgrimage, and whipping (We can not stress the whipping enough). Hilariously (if you're into that sort of thing), these pilgrimages probably helped the plague spread faster.
The aforementioned guys with whips (known as Flagellants and hailing originally from Germany) figured out one thing pretty quickly: God loved them, so the cause must be poison. Poison spread either by Jews or Lepers or Muslims or a combination of all three. One of the most beloved explanations for the poisonings was that the Muslims had found the Ark of the Covenant and wanted to give Israel back to the Jews, who would trade them either France or Spain and had then hired out the Lepers to poison the wells of Christians in an effort to kill them all (and, depending on where one was, eat their children). When local authorities (both church and government) tried to point out that the Jews and Lepers weren't exactly, say, not dying of the plague, they were usually ignored. When the local authorities tried to point out later that killing all the Jews and Lepers hadn't, say, stopped the plague in any way, they were usually ignored again. Mostly because their subjects had died pretty horrifically.
After deciding the cause of the disease, the afflicted was offered a huge number of cures, all of which were just... just awful.
Lance the Buboes
This is what they're cutting. This. With a rusty, dull knife.
This treatment begins and ends with horror. First, the local barber-surgeon (a medical profession encompassing hair cutting, dental work, and stabbing your organs) will take his trusty knife and stab it into that horrible, painful, tumor-like swelling on your neck. If you're lucky he'll get you roaring drunk beforehand, but no promises. Your "surgeon" will collect any blood or pus that comes out, and cover the cut in a mixture of lilies, tree resin, and dried human shit.
Yes, you read that right. Not only were these people covering your open wound in dried human shit (it bears repeating), but they were keeping human shit around long enough to let it dry.
Working under the assumption that too much of one of the four humours was bad (the four humours being blood, snot, stomach bile, and shit), medieval doctors' typical solution was to either gash your arm open and let all your blood out or to just positively cover you in leeches. This "cure" has the duel bonus of not helping at all and weakening the Hell out of your immune system. Bleeding is probably what killed George Washington, and you know that if it killed the great American Hulk no medieval peasant ever had a chance.
Many medieval plague pamphlets recommend that both animal and human corpses be removed from the streets and public areas. The fact that this wasn't already being thought of is infinitely more horrifying than the cure itself.
Roast egg shells and crush them into a powder, add flower parts, and toss all of this into a pot of good ale. Heat up the ale. Drink this twice a day. Not only does this "Cure" not work, it ruins alcohol twice a day. Twice!
Medieval Doctors: "I don't even know anymore. Just hold this."
Not only was magic considered a legitimate cure, its solutions were just as horrifying as what the doctors were doing (though, in a lot of cases, the doctors were the ones administering it). The first step was to hold a live chicken. Just hold it for a while. The next step was to drink your own piss twice a day. At this point we'll take the hot ale with egg shells
Not all of these cures were actually bad, lancing buboes is a relatively effective treatment, though covering those open wounds with shit didn't exactly help. Getting the corpses out of the street was completely effective, as removing dead animals and people got rid of a good deal of the rats carrying infected fleas, removing the main form of early transmission. Interestingly, the king of England at the time, along with most doctors, assumed that the infecting cause was the bad smell, as opposed to the... you know, corpses. In most cases, the civic response was to try to improve the smell of the corpses and fecal matter, which usually involved just carrying flowers while stepping over the shit. This was significantly less effective. Drinking urine is essentially neutral, until you remember that you're, you know, drinking your own urine.
At least the doctors weren't storing that... Mostly