As mentioned, you may not show any symptoms of an STD or the symptoms may be so mild that they are overlooked. However, if an STD is left untreated, severe complications could occur.
Possible complications include:
Pelvic pain
Pregnancy complications
Eye inflammation
Arthritis
Pelvic inflammatory disease
Infertility
Heart disease
Cancers, such as HPV-associated cervical and rectal cancers