1) Give your healers more than the Basic Healer Personality. The majority of the fantasies I read have healers with the following qualities:
-self-sacrificing to the point of stupidity
-brusque with their patients who don't obey rules, but ultimately ineffective when the heroes need to get up and do something.
And that's it.
Come on. Healers are people too. They must occasionally get pissed off, especially when they've had very little sleep and there are people dying all around them and those goddamn stupid "heroes" are getting out of the bed with broken ribs again. Let them scream and rant at times. Let them break down crying. Let them rest and get irritated when something interrupts that rest. The best heroes are allowed that, and even secondary characters when the author really knows what she's doing. Why should the healers be the only stock characters in a fantasy?
Let them have other less fluffy qualities, too. Why are healers forever rushing in and healing anything and everything without thought of payment? How are they supposed to make a living? A mercenary healer who knows his value and is dickering with the village to come in and cure the results of a dragon attack would at least be a different character to read about. What about someone who was originally a soldier, finds out that he has the healer's gift (which in a lot of fantasies is an innate magical ability), and gets hauled in for training because his people need healers so desperately? I don't imagine that he would be best pleased, and the usual healer claptrap about violence being Wrong wouldn't sit well with him, either. Convince him that death is an enemy, and he might attack it like that enemy instead of crooning New Age stuff.
Healers don't need to be all fluffybunny stereotypes, which it sometimes seems like they are.
2) Vary their bedside manner. Even if you manage to get the former soldier to use his magic, I don't think that he would discover he has a heart of gold and just melt around everybody. Maybe after a very long time in this different environment, but three days, or three months, shouldn't be enough to divest him of his former way of dealing with people. He could be uneasy with patients who are crying out in pain, or want to treat them like he would a wounded comrade on the battlefield. If he wasn't good with children, he wouldn't miraculously become so. If he's blunt and someone's being stupid, climbing out of bed because she thinks she has to save the world, he might not have any hesitation about tying her to the bed.
Beyond affecting their personalities, the fluffybunny claptrap infects every dealing that fantasy healers have with other people, even when they are the main characters in their own stories. They're forever anxious about their patients instead of themselves, so understanding it's possible to be sick from reading about them, and if they didn't want to be healers at first, they're inevitably shown the error of their ways. There's something about the word "healers" that gets into authors' heads and conjures images of white robes and gentle smiles, making them forget that these are people who might be up to their elbows in blood and pus.
Maybe they should start calling them "doctors" instead? Most people don't seem to have trouble believing that doctors are varied in personality.
3) Remember the damn wounds. It doesn't happen as much with diseases, although it can, but fantasy authors have a way of putting their characters through wounds and then forgetting that they have them. Wolf bite on the arm? Doesn't matter! The character is swinging a sword as merrily the next day as if nothing ever happened. Sprained ankle with torn ligaments? It exists only to provide some worry for the other characters. She'll be using crutches by the second day, bouncing up and down on it by the fourth. (No. Rather not. I've been through that, and it took four days for me to start walking, even as slowly and painfully and with as much support as I did. The ankle is still weaker than the other one. It wouldn't be even that swift in a world with less advanced medical technology).
If the character does extensive running, walking, and fighting, the wound should tear open. If it isn't tended and cleaned properly, the bandages changed every so often, then the character could sustain an infection, or gangrene, or loss of the limb. It's the rarest of fantasies where someone actually has to amputate a limb, but it should happen more often with as much as fantasy heroes get injured and then ignore the injuries.
Some fantasy authors do remember, but it's more in the sense of saying, "Oh, well, that's why we have a healer!" Which leads to...
4) Resist the temptation to use the healer as a magical cure-all. A lot of fantasies show healers as drawing the pain into their bodies, and leaving the person who was wounded or sick completely well. The healer is also well after maybe a little rest and food.
Why doesn't anyone realize that drawing pain into your body would fucking hurt?
Magic shouldn't be without cost, whether that cost is in energy, lives, a natural resource, the magic-user's sanity, or whatever. Many newer fantasies that I've read are more conscious of that than the older ones are, which gives me hope. A combination of authorial ignorance and the fluffybunny stuff seems to shield them from questioning healing magic, though. The healer is represented as able to perform miracles because she's wonderful and compassionate and has such a pure spirit she can simply consume the pain and...
Urp. Excuse me. Weak stomach.
At the very least, if the healer is going to suffer what the other character was suffering, the party should now have a devastated healer on their hands. It might make sense to do that in some situations, but if they're going to be held up by wounds or sickness from fleeing cross-country, how is it better to have their healer helpless than it was to have the originally hurt character helpless? Above all, what happens if someone else gets wounded? They'll need the healer then.
It would make a lot more sense to have the healer ease the pain as much as she could, or accelerate the healing process, or whatever metaphor makes the most sense with the magical system you've set up, rather than cure everything. That satisfies your readers that the wounded or sick character isn't tottering on the edge of death, and also that you aren't trying to have the drama of a wound with a deus ex machina refusal to let it actually inconvenience that person. (A lot of authors seem to be deathly afraid of inconveniencing their characters. That may be the subject of a future rant).
5) Be as specific as you can. Other miracles in fantasy healing are attributable to "herbs." There is nothing those damn things can't do. Worried that your character might die from an incurable poison? Just have the local hedge-witch dash out into the woods and find some "herbs," and everything is fine again. No mention of why the poison is a threat at all if the herbs are that common and that easy to prepare.
hellga has very nicely provided some references for real-world herbs, in a comment to the previous rant. Do some research, and find out what the hedge-witch is dumping into the brew or poultice or whatever it is that she's making. Or make up a magical herb. Many fantasy readers will excuse the author making up some new flora and fauna, just as they'll accept new religious beliefs, languages, races, and the rest.
Once again, though, stop short of using that herb to cure everything. Give it some limitations. Maybe it cures the poison, but permanently weakens the character thereafter. Maybe, if it's magical, it causes imbalances in the character's magic. Maybe it's so valuable that it's been overharvested, and now people are competing to find and use it. Maybe it's addictive. Some fantasy authors go crazy and make herbs that cure absolutely everything. A tip: Just because it's a plant doesn't mean it can't be a deus ex machina.
6) Think about the bad consequences of healing magic. Healers in fantasy are usually portrayed as being very familiar with pain, poisons, death, wounds, and so on. They can often travel inside the person's body and change things around to suit themselves.
And no one is ever afraid of these people?
A healer gone bad could be a very interesting villain. If he could spread sickness to an enemy through a touch, and then got called in to "heal" the sick person,
who is going to know the difference when that person dies? If no one else in the village has such knowledge of poisons, will anyone be able to catch him? If he knows pain inside and out, what's to stop him from reversing his gift and causing people agony if he can reach them, or make eye contact with them, or whatever he ordinarily does to make them feel better?
This is probably the kind of thing that that fluffybunny claptrap is designed to hinder. If only compassionate people can be healers, runs the idea, then no healer can possibly be evil. His magic would just stop working for him.
But why limit yourself this way? For one thing, that brings the pink unicorn-ish tinge back into the magic, when fantasy authors have enough trouble with that in the first place. For another, it often doesn't work with the magical system of the world; every other kind of gift can belong to good and evil people both, since it's just power, but the healing magic is the exception for no reason. And for a third, it arbitrarily cuts out one kind of person as an antagonist and pushes them towards becoming a stock character. (Come to think of it, that may be why so many fantasy healers have the Basic Healer Personality).
But say you're determined to cling to this. Healers are good and powerful and wise and kind people all the time.
Then why aren't they targets?
One potentially really nasty plotline is to return to the Dark Lord who can create diseases, but for whatever reason, can't create a perfectly fatal one, or can't target the hero. So why doesn't he send a different sickness first, one that attacks the healers and eats their healing magic? Then, when the less-than-perfect plague comes along, the people who have become used to depending on healers to cure them are as helpless as babies.
Or the healers could be good people, but still in trouble if they can't cure everything. People can become desperate in times of plague, or even in times of war, and start looking for someone to blame. If the healers can't protect them any longer, and don't have much to defend them beyond their insistence that they're Good--no ability to strike out in violence or twist their gifts to cause pain, for example--then they might be the ones on the stakes and the pyres.
I seem to start thinking like a Dark Lord when I write these rants. But every fantasy author has to, to a certain extent, or kill her own suspense, a huge problem in some subgenres.