My major criterion is that I need it to be something of the sort where a nineteenth-century doctor would prescribe a change of climate as his primary treatment. The patient was sent to "take the sea waters" on a warm island, but hated that and claimed it made him worse, so now he's taking a trip to a cold and mountainous region. Whether this is beneficial to him, detrimental, or completely ineffective depends on the disease or condition I choose; the story can accomodate any of the three.
The obvious choice is consumption/tuberculosis, since I know nineteenth-century doctors sent patients all over the place in the hopes of improving their health. (Mostly to places with dry air, like deserts, but mountains are also an option -- and really, it ain't hard to justify any climate within the bounds of nineteenth-century quackery.) I've already written a book with a consumptive character in it, though, and would like to not go back to that particular well just yet.
My second thought was malaria, since this character has traveled to tropical regions. It seems unlikely, though, that his doctor would have sent him to a warm island if he was suffering malarial relapses. And I may be giving a *different* character malaria later on in this series, so it might be good to save that. :-)
I'd love suggestions of other chronic conditions that were recognized in the nineteenth century. I've done a fair bit of research on the period, but the medical focus has usually been on acute diseases like cholera, measles, diphtheria, typhus, smallpox, etc, or else conditions of childhood malnutrition like rickets. Gout? Venereal disease of some sort? The guy isn't a womanizer, but it doesn't take a lot of sleeping around in that day and age to pick something up. Any details on how a given disease or condition was viewed in Victorian England, and what treatements were prescribed for it, would be appreciated.