Sexually transmitted infections (STIs) can be caused by bacteria, viruses, or other organisms. Common STIs include chlamydia, gonorrhea, herpes, human papillomavirus (HPV), and HIV. Many STIs have no symptoms but can cause health problems if left untreated, such as infertility, ectopic pregnancy, or increased risk of HIV transmission. Using condoms correctly and limiting sexual partners can reduce the risk of contracting an STI.