Virus: A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria. Virus was discovered by Dmitri Ivanovsky in 1892.
Protozoa: It is an informal term for single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris.
Bacteria: It is a type of biological cell, which constitute a large domain of prokaryotic microorganisms. Bacteria are microscopic living organisms, usually one-celled, that can be found everywhere. Bacteria was discovered by Antonie Van Leeuwenhoek.
Nematode: They are multicellular insects with smooth, unsegmented bodies. They feed on plants are so tiny that you need a microscope to see them. Nematode also called Roundworms.
Disease caused by Virus
Measles, Mumps, Smallpox , Rubella, Chickenpox, Hepatitis, Polio, Rabies , Ebola and Hanta fever, COVID-19, etc.
Disease caused by Protozoa
Malaria, Sleeping sickness, Chagas Disease, Dysentery, Leishmaniasis, etc.
Disease caused by Bacteria
Tuberculosis, Tetanus, Diphtheria, Whooping cough, Typhoid, Cholera, Leprosy, etc.
Disease caused by Nematode
Filariasis, Ascariasis, and Trichinosis
Hence, Smallpox and Rabies diseases are caused by the virus.