Originally, what would be called today a "health
spa", a sanitarium offered special
diets (mostly
vegetarian, sometimes based around a house specialty, like
cereal or a certain local
fruit),
excercises, and sometimes
hydrotherapy and other, more arcane devices in a (more-or-less) hygenic atmosphere. Usually these were situated in small towns, or at least outside of cities --
19th century air pollution was worse than it is now, due to
coal smoke, and what with all those
horses crapping everywhere, and the relative lack of
indoor plumbing,
coliform bacteria and airborne allergens tended to excerbate any preexisting respiratory problems -- and were specially indicated for people with
tuberculosis and "nervous afflictions", which could range from everything from mild fatigue to full-out
schizophrenia. As time went by, many "sans" began to specialize in "lungers", madmen, or what would now be called "
detox", with the same establishment often changing focus over time as treatments did. (The advent of
antibiotics sounded the death knell for the consumption industry.)
Nowadays, most of these places are now either detox-oriented, resorts, or torn down: ironically enough, health spas are booming, and a replica of one of the most famous of the old-time sans, Hotel Bellevue (which was the setting of Thomas Mann's Magic Mountain) has been doing land-office business in Palm Beach under the name "White House Hotel".
Plus ca change... I suppose.