Sexually transmitted infections (STIs) are caused by bacteria, viruses, protozoa, or ectoparasites transmitted through sexual contact. Core management principles: (1) Syndromic approach: treat empirically based on clinical syndrome (urethritis, cervicitis, genital ulcer, PID) while awaiting confirmatory testing, since treatment delay increases transmission. (2) Partner treatment: expedited partner therapy (EPT) allows treating partners without clinical evaluation — legal in most US states for chlamydia and gonorrhea. (3) Co-testing: always test for both chlamydia AND gonorrhea (frequent co-infection); always offer HIV and syphilis testing. (4) Antimicrobial resistance: N. gonorrhoeae has developed resistance to nearly every antibiotic used; current CDC guidelines (2021) recommend ceftriaxone 500 mg IM × 1 dose (increased from 250 mg) as monotherapy (azithromycin no longer routinely added unless chlamydia co-infection present). Key organisms: Chlamydia trachomatis (most common reportable STI — often asymptomatic), Neisseria gonorrhoeae (increasing antibiotic resistance), Treponema pallidum (syphilis — resurgent), HSV-1/HSV-2, HPV (most common STI overall), Trichomonas vaginalis, HIV. Reporting requirements: chlamydia, gonorrhea, syphilis, and HIV are reportable to public health in all US states.