Protozoa, derived from Greek words meaning “first animal,” are traditionally chemoorganotrophic protists. They are unicellular microorganisms without cell walls, often free-living or parasitic, and typically aerobic. As eukaryotic cells, they possess a true nucleus and can be microscopic or visible to the naked eye. They move using pseudopodia, flagella, cilia, or direct cell movements and can have various nutritional modes, including autotrophic, heterotrophic, and saprozoic. They inhabit aquatic or terrestrial environments and can be free-living or symbiotic. Reproduction occurs both asexually (fission, budding, cysts) and sexually (conjugation, syngamy). Protozoa reproduce asexually through fission, budding, or multiple fission (schizogony), and sexually via conjugation. In fission, a cell divides evenly; in budding, division is uneven; in multiple fission, the nucleus divides multiple times before cell division. Conjugation involves exchanging haploid micronuclei, leading to genetic variation. Ciliates, a type of protozoa, have micronuclei and macronuclei, controlling genetic variation and everyday cell functions, respectively. Protozoa obtain nutrients through absorption, ingestion, or engulfing food. Absorption involves nutrient uptake across the plasma membrane. Ingestion uses cilia to move food into a cytosome. Engulfing, or phagocytosis, involves surrounding food with pseudopods and digesting it within vacuoles. While many protozoa are beneficial, some cause diseases in humans, including malaria, African trypanosomiasis, amoebiasis, giardiasis, Chagas disease, leishmaniosis, toxoplasmosis, and cryptosporidiosis.

Viruses are small, obligate intracellular particles visible mainly with an electron microscope. They lack the machinery for energy generation and synthesis of large molecules, making them obligate intracellular parasites. Viruses have an inner core of nucleic acid surrounded by a protein coat and cannot grow on artificial media but can grow in animals, eggs, or tissue cultures. They lack cellular organization and essential enzymes for protein and nucleic acid synthesis, and are unaffected by antibacterial antibiotics. Viruses are ultramicroscopic, with sizes ranging from 20 nm to 450 nm, and have a protein shell (capsid) surrounding the nucleic acid core, which can be DNA or RNA but not both. They multiply by hijacking the host cell’s genetic material, leading to new virus assembly.

Structurally, viruses consist of a core (genomic material, either DNA or RNA), an envelope (in some viruses like HIV and influenza), capsomeres (repeating protein subunits), and a nucleocapsid (combined core and capsid). Some viruses contain enzymes essential for infection, such as lysozyme in bacteriophages and reverse transcriptase in retroviruses.

Viruses exhibit three types of symmetry: helical, polyhedral (icosahedral), and complex. Helical viruses, like tobacco mosaic virus, have a helical structure. Polyhedral viruses, like adenovirus, have 20 triangular faces and 12 vertices. Complex viruses, like poxviruses, have unique structural components.

RNA viruses have RNA genetic material and are typically single-stranded, while DNA viruses have DNA genetic material and are usually double-stranded. RNA viruses mutate faster and replicate in the cytoplasm, whereas DNA viruses replicate in the nucleus. Viruses infect all cellular life forms, including bacteria (bacteriophages).

Viral replication involves five steps: adsorption (attachment to host cells), penetration (entry into host cells), synthesis (new viral components synthesis), maturation (assembly of new virions), and release (departure from host cells). Bacteriophages exhibit lytic (virulent) and lysogenic (temperate) cycles. In the lytic cycle, phages cause host cell lysis, while in the lysogenic cycle, the phage DNA integrates into the host genome without causing lysis.

Common viral diseases in Ethiopia include HIV/AIDS, malaria, African trypanosomiasis, amoebiasis, giardiasis, Chagas disease, leishmaniosis, toxoplasmosis, and cryptosporidiosis. Understanding these diseases helps in their prevention and treatment