The plural form of heliotherapy; multiple treatments or healing methods that use sunlight or solar radiation to cure illness or improve health.
From Greek 'helios' (sun) and 'therapeia' (healing, medical treatment). The plural '-ies' follows standard English noun pluralization patterns.
Before sunscreen became fashionable, sun exposure was actually prescribed as medicine for tuberculosis and rickets—heliotherapies were so trusted that hospitals built special 'sun porches' just for treatment.
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