Sexually transmitted infections (STIs), also known as sexually transmitted diseases, are transmitted through direct intimate contact with an infected person. The causative organisms are viral, bacterial, or protozoan and can coexist. Approximately 15 million new cases are diagnosed annually, with the incidence of STIs peaking during adolescence. The prevalence of viral STIs exceeds that of bacterial STIs. Some STIs are uncomplicated, such as with Trichomonas infection, but others, such as chlamydia and gonorrhea, if left untreated, can lead to long-term health problems such as pelvic inflammatory disease, infertility, chronic pelvic pain, and ectopic pregnancy.