An untreated substance is not cleaned and has not had special substances added to protect it or make it safe to use.

untreated water/sewage/waste 未经处理的水／污水／废弃物

This wood is vulnerable to water damage because it's untreated. 这种没有处理过的木头遇水会受损。

Untreated illnesses, injuries, people, or animals do not receive medical treatment.

untreated illness/disease/depression 未治疗的疾病／疾病／抑郁症

It is a form of anaemia which is nearly always fatal if left untreated. 这是一种贫血病，如果不进行治疗的话会有生命危险。