The study of organisms too small to be seen with the naked eye including bacteria, viruses, fungi, algae, protozoa, and helminths. Procaryotes are small, unicellular organisms. Eucaryotes may be unicellular or multicellular. They are generally much larger than procaryotes.
Viruses are parasites of cells.
Viruses are not considered to be alive because they are not composed of cells. Sometimes there is also a lipid bilayer around the coat. Viruses can not replicate outside of a living cell. They are obligate intracellular parasites. RNA that causes disease in plants.