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(A virus is) “a piece of bad news wrapped in a protein coat.”

—Peter Medawar


Viruses are the smallest form of replicating intracellular microorganisms that are comprised of sets of genes either DNA (DNA viruses) or RNA (RNA viruses) packaged in a protein coat, capsid (naked capsid viruses) or in a nucleocapsid/capsid, and an outer lipid bilayer envelope (enveloped viruses). Viruses have spikes on their outer surface that bind to the receptors on host cells and antibodies generated against the spikes neutralize the virus. Viruses are dependent upon host structural components and metabolic functions. DNA viruses replicate in the nucleus by using host RNA polymerase for transcription and either host or viral DNA polymerase for replication (exception are poxviruses that replicate in the cytoplasm). On the other hand, RNA viruses replicate in the cytoplasm using its own viral RNA-dependent RNA polymerase for both transcription and replication (exception are influenza viruses and retroviruses that replicate in the nucleus). Naked capsid viruses are assembled inside the cell and released upon cell death, whereas enveloped viruses acquire lipid bilayer membrane mainly from plasma membrane and in some cases from nuclear or cytoplasmic membranes. Viral-infected cells may result in cell death and tissue damage (pathology) generally seen in acute infections; however in many cases, the viral infection persists in hosts causing a chronic or latent infection with little or no pathologic changes in target cells or tissues. Since most viruses use their own enzymes (RNA or DNA polymerases) which could be a target for antivirals, they are prone to genetic changes due to lack of proofreading ability of these enzymes. The major genetic changes that viruses undergo are mutation and recombination that allow viruses to escape the immune response and cause damage or persist in the host. During viral latency, viral genome persists in host and may not be eliminated by antiviral drugs. It is difficult to develop strategies to eliminate latent viral infections by antiviral drugs.

A virus is a set of genes, composed of either DNA or RNA, packaged in a protein-containing coat called a capsid. Some viruses also have an outer lipid bilayer membrane external to the capsid called an envelope. The resulting complete virus particle is called a virion. Viruses have an obligate requirement for intracellular growth and a heavy dependence on host cell structural and metabolic components. Therefore, viruses are also referred to as obligate intracellular parasites or microorganisms. Viruses do not have a nucleus, cytoplasm, mitochondria, or other cell organelles. Viruses that infect humans are called human viruses, but are considered along with the general class of animal viruses; viruses that infect bacteria are referred to as bacteriophages (phages for short), and viruses that infect plants are called plant viruses.

A virus is an intracellular microorganism containing DNA or RNA genome and a protein coat, and, in some cases, a lipoprotein envelope

Virus reproduction requires that a ...

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