In this case, the United States. When healthcare is expensive and hard to access, not everybody gets it.

Syphilis symptoms can be so mild they go unnoticed. When you combine that with risky sexual behavior (hook-up culture, anti-condom bias) and lack of testing due to inadequate medical care, you can wind up with untreated syphilis. If you become homeless, care gets even harder to access.

You get diagnosed at a late stage when treatment is more difficult. They put you on a treatment plan, but followup depends on reliable transportation and the mental effects of the disease have made you paranoid. Now imagine you're also a member of a minority on which medical experiments have historically been done without consent or notice.

You don't really trust that those pills are for what you've been told at all. So difficulty accessing healthcare, changing clinics as you move around with medical history not always keeping up, distrust of the providers and treatment, and general instability and lack of regular routine all add up to only taking your medication inconsistently.

Result: under-treated syphilis