IN GERMANY, Are there any individuals or groups who are not covered under health insurance?

Are there any individuals or groups who are not covered under Germany's system? Into what categories do they fall, and what percentage of Germany's total population do they constitute?
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Just to be precise Mbrcatz's answer does not apply to - tourists or workers from any other EEA country (inc Switzerland) - serving members of NATO forces (not families) as they are covered by reciprocal health care agreements (RHCAs) Germany also has bilateral agreements in place with certain countries under which visitors are covered , but not as extensively as some other European countries such as the UK.
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