Many STDs can cause very severe complications if they are not treated.
For example, if gonorrhea is not treated it can spread to your joints, skin, and even the retina of your eyes.
Girls that get chlamydia or gonorrhea and don't know it, or don't get it treated, can develop serious infections of their reproductive organs that can prevent them from having children.
If syphilis is not treated it can affect your heart, your brain and other organs.
Women and men who have sexual infections that are not treated can pass them on to other people.
Finally, women with untreated sexual infections can pass them on to their newborn babies, who can develop mental retardation, blindness, infections in their joints, or even die.
It is very important that people with STDs get treated as early as possible.