Most parasitic infections cause gastrointestinal symptoms. Symptoms by infection type include: Trichomoniasis: This sexually transmitted disease caused by a parasite often has no symptoms. Itching, redness, irritation, and an unusual discharge in the genital area are some symptoms.Giardiasis: Diarrhea, gas, upset stomach, greasy stools, and dehydration.ymptoms by infection type include: Trichomoniasis: This sexually transmitted disease caused by a parasite often has no symptoms. Itching, redness, irritation, and an unusual discharge in the genital area are some symptoms.