Indeed, unlike any microorganism, many viruses can, in suitable cells, reproduce themselves from their genome, a single nucleic acid molecule i.e., their nucleic acid alone is infectious. They replicate only within cells of the host that they infect. Furthermore, since viruses have no ribosomes, mitochondria, or other organelles, they are completely dependent on their cellular hosts for energy production and protein synthesis. Viruses, on the other hand, are smaller and simpler in construction than unicellular microorganisms, and they contain only one type of nucleic acid-either DNA or RNA, never both. Microorganisms grow by synthesizing their own macromolecular constituents (nucleic acid, protein, carbohydrate, and lipid), and they multiply by binary fission. They always contain DNA as the repository of their genetic information, they contain RNA, and they have their own machinery for producing energy and macromolecules. These microorganisms, however small and simple, are cells. The unicellular microorganisms can be arranged in order of decreasing size and complexity: protozoa, fungi, bacteria, mycoplasmas, rickettsiae, and chlamydiae. In the simpler viruses, the virion consists of a single molecule of nucleic acid surrounded by a protein coat, the capsid the capsid and its enclosed nucleic acid together constitute the nucleocapsid. The filtration studies has shown that virus particles (virions) range from about the size of the smallest unicellular microorganisms (300 nm) down to objects little bigger than the largest protein molecules (20 nm).

The new group of microorganisms are known as the filterable viruses.

Outside a susceptible cell, the virus particle like a bacterial spore is metabolically inert on the other hand, when replicating in a cell, it exhibits all the characteristics of life. Unlike any microorganism, many viruses can, in suitable cells, reproduce themselves from their genome, a single nucleic acid molecule, that is, their nucleic acid alone is infectious.

As viruses have no ribosomes, mitochondria, or other organelles, they are completely dependent on their cellular hosts for energy production and protein synthesis. Viruses are smaller and simpler in construction than unicellular microorganisms, and they contain only one type of nucleic acid-either DNA or RNA-never both.

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