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Viruses are recognized as the smallest type of microorganism, and this classification is based on their unique structure and size. Unlike bacteria, fungi, and protozoa, which are all cellular organisms with more complex structures and larger sizes, viruses are non-cellular entities. They consist primarily of genetic material (either DNA or RNA) enclosed in a protein coat, known as a capsid. Some viruses also have an outer lipid envelope.
The size of viruses typically ranges from about 20 to 300 nanometers, making them significantly smaller than bacteria, which generally range from 0.5 to 5 micrometers. Fungi and protozoa are also larger than viruses, as they are more complex organisms containing cellular structures that allow them to grow and replicate independently.
Because of their small size and simple structure, viruses can only replicate inside the living cells of a host organism, which further distinguishes them from other microorganisms. This unique attribute highlights the significant differences among various types of microorganisms and clarifies why viruses are classified as the smallest among them.