As can be read on the Wikipedia: "A preventorium was an institution or building for patients infected with tuberculosis who did not yet have an active form of the disease. Popular in the early 20th century, preventoria were designed to isolate these patients from uninfected individuals as well as patients who showed outward symptoms."
http://www.flickr.com/photos/postaletrice/3231685557/

I don't think it's solely for children though.


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Note added at 21 mins (2009-11-19 09:51:54 GMT)
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Having said that, most references seem to pertain to children.

For the majority, the hospital served as a preventorium (meaning sanatorium care to prevent TB). Most of these children had been exposed to the disease, ...
www.feltondesignanddata.com/.../id22.html



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Note added at 8 hrs (2009-11-19 17:41:25 GMT)
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I would leave 'preventorium' in as it's not exactly the same as a sanatorium and it seems to be standard wording in France. I know it's not very common in English, but I have found this Irish site:

the expression "institution" means a hospital, sanatorium, maternity home, convalescent home, preventorium, laboratory, clinic, health centre, first-aid station, dispensary or any similar institution:
http://acts.oireachtas.ie/zza28y1947.2.html

Also:

Ultimately, Dr. Huerta has created a “preventorium”—the opposite of sanatorium—at ... Hospital Center Cancer Preventorium begun in July 1994, ...... some of the unnecessary medical expense. • In the United States, ...
www.nhlbi.nih.gov/health/prof/heart/other/paho/wksp_sum.pdf