The parasite that causes trichomoniasis comes from a person who is infected with it and then transmits that same parasite to the partner through sex.
It's caused by the single-celled protozoan parasite, Trichomonas vaginalis.
Some people who are infected with the Trichomoniasis parasite may not even experience symptoms so it's possible to have a partner with the Trichomoniasis infection and not know it.
Then when you go and have sex with that person you then become infected.
You can also become infected with Trichomoniasis through contact with damp or moist objects such as towels, wet clothing and even toilet seats.