In the early part of the game, when your villagers don't know very much about medicine, villagers can die in what we would consider their 'prime' years. If she doesn't have 'sick' showing in her status, your doctor can't help her. Research is a good activity for her, since it's not likely to cause injury (a possibility with some activities). I've seen even a villager in the 'red zone' improve, though, when I have a good amount of food in the bin and it's increasing. Cross your fingers!