Viruses are small particles that contain short sequences of nucleic acid. They can be ribonucleic acid (RNA) or deoxyribonucleic acid (DNA). In most organisms, DNA has a double-stranded structure based on Watson and Crick pairing. Viruses, however, have a unique property: their DNA or RNA can be either double-stranded or single-stranded.
Typically, viruses are enclosed in a protein shell known as the capsid and protected by a lipid envelope derived from host cells. Viruses are also very diverse in their morphology and size. They can be spherical, rod-shaped or filamentous and may have spikes sticking out of the capsid that enable them to bind to host cells and enter.
Some viruses cause diseases in animals and humans, including measles, polio, dengue fever and HIV. Other viruses are benign, such as those that infect bacteria. In fact, it is estimated that trillions of these harmless viruses live inside us at any given time.
Viruses are obligate intracellular parasites; they can’t survive or reproduce outside the cells of living organisms. During the replication process, viruses depend entirely on the complicated machinery of their host cells to produce RNA and proteins for them. Because of this dependency, viruses are considered nonliving organisms by most biologists. Some scientists, however, dispute this definition and argue that viruses can perform many of the same functions as living organisms, such as converting food into energy. In addition, they can replicate in the right conditions. This gray area makes some people uncomfortable and leads to a lot of controversy around viruses.
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