Which sexually transmitted infection is known to cause infertility if left untreated?

Gonorrhea and chlamydia are known to cause infertility if left untreated due to their potential complications. Both these infections can lead to pelvic inflammatory disease (PID) in women, which can damage the reproductive organs, such as the uterus, fallopian tubes, and ovaries. This damage may result in severe scarring and blockages, making it difficult for an egg to travel from the ovaries to the uterus or for sperm to reach the egg. In men, untreated gonorrhea or chlamydia can lead to epididymitis, which can also adversely affect fertility.

In contrast, while HIV/AIDS and syphilis have significant health implications, they do not directly cause infertility like gonorrhea and chlamydia can. Herpes Simplex Virus primarily causes lesions and can lead to recurrent outbreaks, but it does not inherently result in infertility. Hence, the direct link between untreated gonorrhea and chlamydia and fertility issues makes this answer the most accurate in the context of STIs affecting reproductive capability.