What are the effects of untreated STDs?

It depends which STD. HIV may develop into AIDS Syphilis eventually affects the brain. Gonorrhoea and chlamydia may make you infertile.
What will happen if an std is left untreated depends on the std.

One cures itself. It would go away if people with it would not have sex or use condoms.

Another gradually eats away the Brain before it caused death.

Another destroys the immune system before it causes death.

Other stds have different outcomes.

It could become HIV and being defing.

Delayed diagnosis and treatment of STDs can lead to long term problems. You also may pass the STD on to your partner.
No. It could lead to death. STupID quEStiON!!
Sexually Transmitted Infections are developing Sexually Transmitted Diseases which include damage of Urinary Tract System, Fallopian Tubes, Uterus, Vagina, etc. Not-treated diseases could be causes of infertility, serious UT diseases, Vaginal itching and discharge. You can discover effects of each STD in recommended related link.
The infection or disease can get worse and most likely start to hurt. I urge you to go check check out by a doctor and get prescription.