Is a tiered classification framework needed for sexually transmitted infections?

Working definitions of a sexually transmitted infection (STI) exist, but no universally agreed operational classification links those definitions to programme, funding, and surveillance decisions. The boundary of the STI category determines which infections enter surveillance, receive programme funding, and are included in testing and screening. Yet, the fields of microbiology, clinical medicine, and public health each define this boundary differently. Contested conditions, including bacterial vaginosis, vulvovaginal candidiasis, scabies, mpox, and sexually transmissible enteric pathogens, expose the inadequacy of the current binary classification. In this Personal View, we propose a three-tiered classification framework: core STIs, sexually associated infections, and incidental sexually transmissible infections, anchored on four explicit classification dimensions. The framework is intended to guide programme mandates and funding, clinical and diagnostic service delivery, and disease surveillance, while accommodating conditions that might shift between tiers as evidence evolves and as local epidemiology dictates. We also discuss the unintended consequences a formal classification could have for equity, stigma, and resource allocation. A graded classification acknowledging degrees of sexual transmissibility is more useful than a binary classification that forces false clarity.