All three existing species of human trichomonads cause similar immune and histopathological host responses. The elicited crossimmunity causes interspecific competition of trichomonal infections in their typical localizations, which results in very rare simultaneous colonization of the same host by different species. This phenomenon points to the existence of a unique immune disorder or subclinical sensitization, regardless of which trichomonas species is in question. The total percentage of infestations in all three species points to the most widespread parasitosis, i.e. the immunodepression of human beings. Trichomonaemia has a very strong immunodepressive action. All the other agents, including viruses act only as opportunists.