Answer:
The process of "exposing healthy subjects to related or weakened disease agents to protect them from the pathogenic agent is now known as vaccination." But using the syphilis organisms would spread a full-blown serious disease agent. "Mercury and Salvarsan were used to treat syphilis. Although both treatments killed the bacterium, Treponema pallidum, the causative agent of syphilis they also caused rashes, liver damage, loss of limbs and life. These side effects have been attributed to improper handling of the drug."
Reference: Gordon State College. “Microbiology and You.” Microbiology and You: An Introduction, 2019