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Thermal waters have their origins in the depths of the Earth. It is during their long way to the surface that they are enriched with chemical properties and trace elements and they also get their biological properties. Their sources are often exploited because of the properties of these waters and they must be protected to avoid infiltration related to human activities on the surface of watersheds. Depending on the soil from which it originates, water is used for various therapeutic orientations. However, they are true ecosystems with multiple interactions and they all have biological properties that are most often unknown. Thermal waters like all waters on earth (surface water, groundwater, oceanic,…) are rich in microorganisms that are essential and very useful for humans and ecosystems. The properties of thermal waters also come from the microorganisms that live there and which are often very specific to the minerality of these waters. Their diversity can be important and when isolated and grown in the laboratory, we can extract and concentrate the active ingredients to exploit them on an industrial scale. © 2020 Elsevier Masson SAS. All rights reserved.