STI screening is a public health priority because the majority of STIs are asymptomatic and transmission occurs unknowingly. The CDC and USPSTF provide evidence-based screening recommendations targeting high-prevalence populations to identify and treat infections before complications develop. Key screening rationale: (1) Chlamydia and gonorrhea: most infections in women are asymptomatic (70% chlamydia, 50% gonorrhea), but untreated infections ascend to cause PID, ectopic pregnancy, and infertility. Screening and treating asymptomatic infections prevents these complications. (2) Syphilis: congenital syphilis is 100% preventable with prenatal screening and treatment — rates are rising dramatically (10× increase since 2012). (3) HIV: 13% of people with HIV in the US are unaware of their status; early diagnosis allows treatment initiation (reducing transmission) and viral suppression (U=U: undetectable = untransmittable). (4) Hepatitis B and C: chronic infections cause cirrhosis and hepatocellular carcinoma; screening identifies candidates for treatment (HCV is now curable with direct-acting antivirals). Universal screening means screening everyone in a defined population regardless of risk factors; targeted screening means screening based on identified risk factors.