Entamoeba histolytica is a protists which causes amoebic dysentery . it is spread over by contaminated food and water.flies are vectors for this but not host because they are not infected by it only spread it.

Answer:

Host-Parasite Relationship:
In the context of parasitology, the term "host" refers to an organism that harbors another organism, known as a parasite. The relationship between a host and a parasite can vary, with some parasites requiring only one host to complete their life cycle, while others may require multiple hosts.

Entamoeba histolytica:
Entamoeba histolytica is a parasitic protozoan that causes the disease known as amoebiasis. It primarily infects the large intestine of humans and other primates. The life cycle of E. histolytica involves two stages: the cyst stage and the trophozoite stage. The cyst stage is the infective stage, while the trophozoite stage is the invasive stage.

Trypanosoma gambiense:
Trypanosoma gambiense is a parasitic protozoan that causes African trypanosomiasis, also known as sleeping sickness. It is transmitted to humans through the bite of the tsetse fly. The life cycle of T. gambiense involves two hosts: humans and tsetse flies. Humans serve as the intermediate host, while tsetse flies serve as the definitive host.

Taenia solium:
Taenia solium is a parasitic tapeworm that infects pigs and humans. It causes the disease known as taeniasis in humans. The life cycle of T. solium involves two hosts: pigs and humans. Pigs serve as the intermediate host, while humans serve as the definitive host.

Plasmodium vivax:
Plasmodium vivax is a parasitic protozoan that causes malaria in humans. It is transmitted to humans through the bite of infected female Anopheles mosquitoes. The life cycle of P. vivax involves two hosts: humans and mosquitoes. Humans serve as the intermediate host, while mosquitoes serve as the definitive host.

Conclusion:
Out of the given options, only Entamoeba histolytica (option A) has only one host, which is humans. It does not require any intermediate or definitive host to complete its life cycle. On the other hand, Trypanosoma gambiense, Taenia solium, and Plasmodium vivax require multiple hosts to complete their life cycles.