German medical troops were typically (but not always) associated with the Deutsches Rotes Kreuz (DRK) which was the German Red Cross. Sanitaters were combat medics. They had a strong understanding of first aid and they were tasked with supporting wounded troops. In addition to Sanitaters there would also be a number of Krankenträgers or Stretcher-bearers. These men woudl have less knowledge than the traditional sanitater but would have a basic understanding of first aid. Behind the lines there would be fully licinsed and trained medical personel such as doctors and nurses. The wearing of medial insignia varied greatly from time to time and place to place. The Soviets generally did not respect the Geneva Convention and so they treated medical personnel as viable targets. On the Western front, visible red crosses were generally respected by the wester allies and medical troops were not fired upon in most circumstance. 

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For more information please see this article. http://www.dererstezug.com/TheSANI1.htm  

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These photographs were taken from a number of online sources and auction sites.  They are presented here for educational purposes.