Apparently one of the reasons that trichomoniasis
infections can be so damaging is that sometimes people aren't just infected with the protozoan parasite that causes the disease... they're also infected with a virus that many of the parasites are infected with. A study recently published in PLOS One
found that the parasite's own infection with a virus known as Trichomonasvirus is one of the reasons that it can cause such a strong inflammatory response. Although the virus doesn't infect human cells, perse, it's presence is an irritant that stimulates the immune system beyond the effects of the parasite alone.
That enhanced inflammatory reaction may explain some of the reason why women with trichomoniasis are more susceptible to infections such as HPV and HIV. It may also explain some of how trichomoniasis can contribute to serious long-term health problems, such as pelvic inflammatory disease and infertility... and why treatment with metronidazole can cure the parasitic infection but does not necessarily improve reproductive outcomes.