Isso é explicado em Wikipédia que o mito de Wendigo, que envolve comer A carne humana é uma cura para doenças e feridas.
A temporary commander is assigned to the fort and to Boyd's horror it turns out to be Colqhoun, now cleaned up and calling himself Colonel Ives. The others still refuse to believe that Ives is the killer especially after he bears no sign of the wounds inflicted on him during the fight at the cave.
Ives tells Boyd that he used to suffer from tuberculosis but when a Native scout told him the Wendigo myth he "just had to try" by murdering him, eating his flesh and in the process curing his illness.
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Boyd and Ives inflict grievous wounds on each other. But they don't die easily due to their recuperative powers.