The phrase "not prone" can be replaced by multiple synonyms depending on the context of use. If you are talking about individuals who are less susceptible to infections, you can use phrases like "resistant," "immune," or "unaffected." For a situation where the person is not likely to engage in risky behavior, you can use "cautious," "conservative," or "wary." In cases where one is not easily swayed by suggestions or opinions, you can use "uninfluenced," "unaffected," or "unswayed." In essence, "not prone" can be substituted with phrases that convey a similar meaning of being unlikely or uncharacteristic to a particular situation.