Final answer:
Gonorrhea and chlamydia can cause infertility primarily via pelvic inflammatory disease (PID) when left untreated, leading to reproductive tract damage in both men and women.
Step-by-step explanation:
Gonorrhea and chlamydia can cause infertility due to the progression of the diseases into more serious reproductive tract infections if left untreated. Specifically, gonorrhea is caused by Neisseria gonorrhoeae and can result in pelvic inflammatory disease (PID) in women and inflammation of the reproductive organs in men, which in turn can lead to infertility. On the other hand, chlamydia, caused by Chlamydia trachomatis, can also cause PID in women and epididymitis in men, both of which can result in infertility if not properly treated.
Therefore, when STDs like gonorrhea and chlamydia are not treated, they can escalate into conditions such as PID, which is a major cause of infertility. Early and effective treatment with antibiotics is vital to prevent these complications. While syphilis and genital herpes are other STIs, they are not the specific diseases in question when referring to the infertility caused by gonorrhea and chlamydia.