The villager you are trying to heal must have a status of "Sick". If they're simply elderly or weakened (or healthy), then trying to heal them will have the results you mention.
If they're weak, you might try giving them some less onerous duties like researching and see if thet helps them get stronger. Having enough food available also makes a difference. If they're elderly, unfortunately, there's not a lot you can do for them except buy more medecine tech which increases longevity.