Sanatoriums (aka health resorts) are wide spread in eastern block countries. They popped up in early 18th century and were used mostly by the upper class. In Soviet times sanatorium resorts became accessible to the working class and served the purpose of keeping the nation healthy.
Nowadays health resorts are quite as popular as SPA and wellness centres - there are more than 200 sanatoriums in post Soviet area and most of them can be found on booking.com.
I took a virtual tour to sanatoriums in Russia, Belarus, Ukraine, Latvia, Azerbaijan and Lithuania and collected photos that have been posted for commercial purposes.