Final answer:
Helminths are not microorganisms due to their multicellular and often visible nature, but their study is included in microbiology because of their microscopic eggs and larvae. Viruses differ from other microorganisms as they are acellular and need a host to replicate.
Step-by-step explanation:
Helminths, which include multicellular parasitic worms, are not considered microorganisms because most of them can be seen without a microscope. However, they are still studied within the field of microbiology due to the microscopic nature of their eggs and larvae, which are involved in disease transmission. On the other hand, viruses differ from other microorganisms like bacteria, fungi, and protozoa because they are acellular, meaning they do not consist of cells, and they require a host to replicate.