A. No symptoms when infected: Many STIs (like Chlamydia or HPV) are asymptomatic, meaning individuals are unaware they are carriers. This leads to unintentional transmission to partners. B. Contaminated birth control devices: Modern birth control manufacturing and clinical insertion (like IUDs) follow strict sterile protocols; this is not a primary driver of high transmission rates. C. Ineffective treatment options: Most common STIs have highly effective antibiotic or antiviral treatments available if diagnosed. D. Contaminated catheters: While catheters can cause Urinary Tract Infections (UTIs), they are not a vector for the transmission of sexually transmitted infections.