I'm very glad that you posted this, because I've been trying to determine the nature of Fanny's complaint. JA mentions Fanny's ill health in several places, but the only actual symptoms described, at least as far as I can recall, are the headaches, the tendency to become rather quickly over-tired by exercise, and her small size when she first arrives at MP, and all of these could be caused by many things. I myself suspect she has some type of respiratory or sinus problem, but migraines are an equally valid explanation.