The most common STIs, grouped by the type of pathogen that causes them: parasitic, bacterial and viral. The chapter covers trichomoniasis, the most common curable STI, and pubic lice, before moving to the bacterial infections chlamydia, gonorrhoea and syphilis. For each one it outlines the cause, how widespread it is and the complications it can lead to if untreated, such as pelvic inflammatory disease or problems during pregnancy. It then turns to viral infections: HIV and how it affects the immune system, HPV and its links to genital warts and certain cancers, and herpes, which has no cure but can be managed.