The Plague is a disease that killed one third of Europe in the fourteenth century, and then a huge chunk of India in the nineteenth century. Just because.
"Holy fuck! Why the fuck is this on my leg?!?" - The Middle-Ages
Bubonic Plague is the most common form of Plague, which is almost good for you, because it's the one with the best chance of survival! That's right, catch this form and you'll only die 60% of the time. The rest of the time you can look forward to permanent disfigurement caused by an enormous, tumor-like bump on your lymph nodes. Oh? You didn't think that was permanent? Oh, but it is. That tumor will stay with you forever. And that picture up there, that's not a shock and awe picture. That's not a 'oh no, this near-groin tumor is totally rare!' picture. Since the 19th century, almost everyone who gets a bubo gets one on or near the groin.
And that's not a little bump either. Bubos hurt. Imagine you're a peasant, you've just recovered from a nasty case of Bubonic Plague and you think that God must have left you behind for a reason. Maybe it's about time to get some humping in, restore the population. After a bit of searching, you find a lady of the night with almost all of her teeth. Yeah, this is it for you, peasant! Big night! And then she happens to graze the horrific, black, throbbing, gurgling (did I mention they gurgle? Because they do, they totally do) boil next to your dick. The next thing you know you're unconcious in an alley and some filthier peasant is trying to steal your one good shoe.
As if the symptoms weren't horrific enough, Bubonic plague is spread by rat fleas. Rat fleas are a lot like human fleas, except that rat fleas have been seen to bite into caterpillars, and suck out their liquified innards. Now, imaginary peasant, keep in mind that you've got one set of clothes in this world, you don't wash them, and you sleep naked in the one room hut you share wth your 12 kids and your cow. Congratulations! Every time you drop so much as a speck of food (when you're lucky enough to have some) you also get to share your lovely home with a pack of enormous black rats. The second those rats start dying of the plague (and the rats always die first) their fleas need somewhere to go. Your dick blood is going to look pretty tasty to those tiny bastards.
And don't let that 40% survival rate up there fool you. One bite from that flea and you've got an even better chance of catching one of the other two forms.
Oh fuck me, that's make-up, right? Please tell me that's make-up.
Pneumonic Plague comes in two varieties, which is going to be awesome for you. There's Secondary Pneumonic Plague, caused when people who catch Bubonic Plague don't get it treated, and see the plague bacteria move up into their lungs. Secondary Pneumonic form is horrifying because it drops your survival rate down from an almost acceptable 40% to a pants-shittingly terrifying 0-5% (which is roughly the chance of being able to pick up chicks at conferences by writing Cracked topic pages on the plague).
Primary Pneumonic Plague is what many historians now think killed the fuck out of Europe during the Black Death. See, first a bunch of people need to get Secondary Pneumonic Plague, and then they need to go outside. Once they're out around other people, they start coughing, and since this is the Middle Ages, we'll be surprised if you don't just go lick their faces outright and hope the box of weeds and rocks you bought from that 'doctor' protects you. Now that those other jerks have started coughing all of that blood and phlegm is in the air and probably on your face, now it's in your lungs. Congratulations! You've skipped Bubonic Plague, and moved right into one of the most horrifying things you'll ever experience.
First, put yourself back in your floppy peasant shoes, yes, that's right, comfy aren't they? You'll notice a fever and a headache, nothing too exciting given your constant, disease ridden state. Until fingers start dropping off it's not a big deal, right? Well, next you'll be enjoying some muscular weakness, again, not a huge deal... And then you start coughing up blood. See, the respiratory symptoms are going to rapidly make this a terrible thing. First, you'll be feeling a shortness of breath, and chest pains. Then a cough, just a little one. And then that cough is going to start bringing up blood. By the end your lungs will be in tatters and, double-plus bonus, you'll also be experiencing bloody anal leakage. Hope you enjoy your straw mat, because no one's going to be coming to collect your body. They all figured out not to get too close to plague-ridden, blood-coughing, corpse people.
Typing "Septicemic Plague" into Google image search just brings up a suicide help line. So here are kittens.
No jokes, this shit is just horrifying. First, the plague bacteria needs to get into your blood in huge amounts, causing a toxic shock. This huge shock of bacteria can be caused by a number of things, a flea bite, bad Bubonic Plague, the phase of the moon, it's really a crap shoot. What you need to know is that you're going to be more plague than human being within about twelve hours. This shock of bacteria is going to cause tiny clots in your blood, and now we get to the fun part. What can you, a filthy, diseased peasant, expect to get from your new disease?
First, you'll get a fever, you'll find that this is about par for the course with every form of plague, but next you'll be enjoying the chills. After that, the abdominal pain, horrific, terrible pain that is the cause of massive internal bleeding. Yes, that's right, since your body won't be able to clot blood normally you'll be bleeding internally and externally, and no cut you sustain will be healing. Your skin will be black and patchy, you'll have tiny little insect bites, and, oh yes, you'll be coughing and vomiting blood. Unlike the other two forms of plague, where you'll be suffering for anywhere up to a week, Septicemic Plague doesn't fuck around. When an epidemic of Septicemic Plague hit Spain people were reported to die within fourteen hours.
Of course, you're not a peasant, and this isn't the Middle-Ages, treatment with modern antibiotics at an early stage drops your chance of dying down somewhere between 4-15%. Assuming you actually catch it in time and your doctor knows what you have. Good Luck!