Or, rules of creating fantasy diseases. Because famines and civil wars and Destiny and Dark Lords aren't enough; what you really need to slam into your fantasy world is an epidemic.
1) Where did it come from? Bubonic plague, which rides in fleas on rats, could be carried on ships when the rats took shelter in them, then leaped off on foreign soil and carried their fleas into contact with new bodies. Smallpox, which devastated Native American populations in the New World, was at times spread deliberately by the Spanish conquistadors, using blankets and other objects which had been in contact with smallpox scabs. Malaria, the disease which has killed more people than any other in history, came from mosquitoes, not bad air as some American settlers thought.
The point here is that if the plague suddenly appears and starts running up and down your fantasy town with a hatchet, you should know where it came from. In general, I would not recommend that the plague be a manifestation of the Dark Lord. For one thing, if he can do that, why does he need to bother with armies? He could just create a perfectly fatal disease and turn it loose. Second, why doesn't the hero ever fall victim to it? You'd think the Dark Lord's top priority would be getting rid of the one person prophesied to defeat him.
2) What is its vector? A vector is something that carries the disease-causing germs along: mosquitoes and fleas, for example. You should know how the disease manages to reach people, as well as what stranger it might have come in with. If the stranger is coughing and hacking, it would make sense for the people he coughs and hacks on to get sick. However, if it turns out that the disease is really carried in elephant urine...well, you're going to have to be pretty creative to say how the stranger could have come in soaked in elephant urine and not have anybody notice.
Fantasy offers the possibility of magical vectors. Perhaps sickness spreads across the land directly following a flight of dragons. Once again, though, some boundaries are appropriate. If one dragon swerves aside from the flight to snatch a quick bird in the air above a village, does that village also get taken? Why does the hero, if he doesn't live in the path of the flight, suddenly have to contend with losing friends and family? (Authors often use sickness to create angst for the hero even when it doesn't strictly make sense to do so). Why, if the dragons are flying straight as an arrow, do these stupid people not realize they're in the way and get out of the way? Okay, maybe the dragons fly too fast for it, but if the path is the same every time and repeated for years, you'd think eventually dunderheads would stop building villages in its shadow.
3) How long does it take to kill? One reason fantasy authors favor sickness is because it allows for that lingering deathbed scene. Wise Old Mentor, coughing and hacking but somehow with a ridiculously clear voice, directs Young Dunderheaded Hero to look under the bed. Lo and behold, there is the carved wooden box with the secret amulet that proclaims him King of Kraaakaaastaaanen. Wise Old Mentor then explains the situation, then dies, leaving Young Dunderheaded Hero in possession of the information, a ton of angst, and the possibility of taking back his kingdom on his own.
Young Dunderheaded Hero wanders outside. Lo and behold, he is also the only survivor of the village, because the Shivering Plague kills in three hours...
Waaait a minute.
Aside from wanting to know why the hero didn't get sick, I want to know how Wise Old Mentor managed to realize in the course of three hours that he was going to die and tell YDH everything important. Something that swoops down and slays that quickly probably isn't going to give him enough time to realize. He might be able to feebly indicate the crown is under the bed, but by the third hour, I would expect delirium and terror, not a clear mind and "I've had a good life" platitudes. It especially doesn't make sense if WOM was sick for three days and everyone else for only three hours.
Keep this consistent. If you want a deathbed scene, create a sickness that doesn't vary so much from victim to victim, or make note that people take variable lengths of time to die.
4) Why is this disease fatal? I've seen a number of fantasy plagues that I couldn't believe would kill anybody. So the person gets a high fever. Why doesn't she recover, as people with high fevers can? For that matter, why is everyone ('cept the hero, of course) dying of it? You'd think one person at least would be able to resist a high fever.
This is the place where you might have to go and do some nasty research. Cholera kills its victims by dehydration, which I didn't know until I looked it up. Cholera victims have diarrhea (which looks like rice water) until they get too dehydrated to survive. Some diseases fill the lungs with liquid that prevents breathing. Others attack the brain. Magical diseases offer a world of really, really nasty possibilities. Perhaps your character is a fire mage. What if the disease wears away at the boundaries of her control, and she dies in a burst of magical flame? Perhaps the disease only attacks people with a certain magical talent, or percentage of non-human blood, or (heh, heh) large amount of Destiny. There's a way to put your hero at risk.
If this sickness is meant to kill, know why it does.
5) What are its demographics? I'm always instantly suspicious when the hero's mother died of "plague" ten years ago, and she's the only one; everyone around him has both parents and tons of siblings. That comes across as a cheap excuse to make the hero an orphan or half-orphan. It gets worse if the author is actually using words like "epidemic" or terms like "the Red Death," which imply a high death rate. Unless the disease really is a magical assassination attempt, it shouldn't only hit the people the hero cares about. (And I still maintain that if the Dark Lord wants to kill him that way, it would be better just to hit him with it, instead of boring the readers to death with angst).
High fatality rate? Then as the hero travels through his country, he should meet a lot of people who lost their loved ones to plague and had their lives changed because of it. Low fatality rate? Then he may meet a few people like that. Everyone except him and his dog? Then he should be moving through a dead country, and the only people he meets should be ones who have a damn good reason to be immune to it, like being of a different species, and I'll be following him around complaining about him surviving inside my head.
6) What are its aftereffects? Scarlet fever, not treated properly, can cause problems with the bones, liver, and kidneys. Smallpox leaves absolutely horrid scars. (Take that, flawless skin of perfect fantasy teenagers). A disease contracted from the bite of an assassin bug is believed to have helped kill Charles Darwin; he was strong as a young man, then declined and spent most of the rest of his life as an invalid. Diseases treated with our antibiotics can clear up, but most fantasy worlds don't have our antibiotics. Anyone who fights a usually fatal disease and survives should be weakened in some way.
Here, you can have fun with magic again. If your fire mage didn't die in the flames of her gift gone out of control but was sick, then perhaps at times she relapses and potentially loses control of her gift, or doesn't dare do any magic for fear of what would happen. I would imagine that during those times, the people she's traveling with look at her pretty uneasily.
7) How do people dispose of the bodies? Mass graves and burnings are often the only practical solutions if a large number of people are dead. I have read a few fantasy books where the heroes buried each body individually. Depends on how many people you have, what the soil is like, and how many days or hours it takes before the bodies start decaying. Even the most dedicated fantasy heroes might start hurrying some of the burials if they're in the middle of a thousand bodies beginning to rot and stinking up the place.
If you're going to burn them, you've got to gather wood, enough to build a huge pyre, and it'll raise one hell of a smoke and a stink. It's possible, depending on the disease that you've created, that that could even help spread the sickness further. Either way, it won't be pleasant, and it shouldn't be. Disease is handled too quickly and too lightly in fantasy most of the time. Make your hero actually walk through the aftermath of one you've made, and he can have all the profound angst he likes--and you get to exercise your descriptive skills in the name of making the reader scream, which are too little exercised next to the ones that make the reader sigh in delight.
8) What is life like for the survivors? Once again, usually treated too lightly. The hero misses his mommy, or both his parents and his siblings if the author went whole hog and killed all his family except for him. There's no mention of how hard he has to labor in order to maintain the farm by himself.
Consider this: One estimate of the bubonic plague is that it killed 25 million people, a third of Europe's population at the time, in just under five years. That led to massive shortages of people able and willing to work, which led to people who could demanding higher wages, which led to the people in power refusing, which led to peasant revolts. The Black Death wreaked incredible damage on people in general, but it didn't just sweep across the land and vanish; its effects reverberated for a long time.
If you have a plague with a 99% fatality rate, you have the scenario for a post-apocalyptic wasteland. Even if it's "only" 50%, that's still higher than the European Black Death was, and its effects should be correspondingly more severe. Who's going to bring in the food? Who's going to hunt? If your plague had after-effects, can people still manage the tasks they used to, and can the mages who may have been victims of a magical disease be trusted? What happens if the sickness hits in the autumn, with winter coming right behind it? The disease's victims probably won't be the only ones to die.
And what about the psychology of the survivors? They’re probably going to wonder why they’re still alive, or why their gods didn’t protect them. They may suffer the effects of severe grief. And if the Dark Lord really is a sadistic monster, or can’t create a perfectly fatal disease, he can march his armies straight through the middle of the weakened villages and plow them into the earth.
More fun ways to put your characters through hell coming up!