Roman Bath
This is kind of a trick question. Hydrotherapy is one of the earliest forms of healing with many unconnected roots.
You can find independent references to this healing practice from several ancient cultures. Some of the cultural traditions include: Rome, Greece, China, Japan, Persia, Hindus, Egyptians, Native Americans, and Europeans. I think that pretty much covers the world throughout time.
Each of these cultures holds their own methods, practices, and beliefs about this healing science, which continues to be practiced throughout the world.
The most documented practices include: steam/sweat bathing, cold water affusion, and contrast bathing. The beliefs driving this practice site hygienic, spiritual, physical and mental disease healing benefits.
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