Third-stage syphilis can potentially be fatal, but this is not absolute. With proper and timely treatment, it can gradually improve. Third-stage syphilis is the most severe and latest stage of the disease, also known as late-stage syphilis. This stage often affects our skin, bones, cardiovascular system, and even the nervous system. When these systems are affected, corresponding complications can arise. If left untreated, late-stage complications can lead to death. However, if treated early, there is a possibility of cure over time.