Final answer:
An almost asymptomatic STI that may lead to pelvic inflammatory disease if untreated is trichomoniasis, caused by the protozoan Trichomonas vaginalis. While symptoms are rare in men, women can suffer serious health consequences without treatment.
Step-by-step explanation:
An almost asymptomatic STI that may cause pelvic inflammatory disease if not treated is trichomoniasis. Caused by the protozoan Trichomonas vaginalis, this infection is one of the most common nonviral sexually transmitted infections (STIs). Although men rarely exhibit symptoms, in women, trichomoniasis can cause symptoms such as urethritis, vaginal discharge, or bleeding, and if left untreated, can lead to more serious conditions such as pelvic inflammatory disease (PID) and an increased risk of cervical cancer.
Pregnant women infected with T. vaginalis also face a higher risk of complications like pre-term delivery. Furthermore, the asymptomatic nature of trichomoniasis in a majority of women highlights the importance of regular testing, especially since it can render them more susceptible to HIV. Trichomoniasis is treatable, so early detection and treatment are crucial for preventing these potentially severe health issues.