For instance, one could have HIV for many years without knowing that was what one had. Syphilis, gone untreated, can last for a long time without notice until its very debilitating effects start to take root. Herpes and HPV, especially the former if it's a strain without warts, which most are, will always stick around in the body and may go unnoticed.

Plus, untreated STIs can result in pelvic inflammatory disease in women, which can create very serious health risks and infertility, but that can also go unnoticed -- or be assumed to be something else by one with it, such as simply bad menstrual cramps -- for a long time.