Updated: Jun 20, 2020
Viruses are minute obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat. Viruses may be observed as transportable genetic elements, commonly of cellular origin and characterized by a long co-evolution of virus and host.
The main function of the virion (which is a complete virus particle) is to deliver its DNA or RNA genome into the host cell so that the genome can be expressed (transcribed and translated) by the host cell. The viral genome, frequently with associated basic proteins, is packed inside symmetric protein capsid. The nucleic acid-associated protein, called nucleoprotein, together with the genome, forms the nucleocapsid. In enveloped viruses, the nucleocapsid is surrounded by a lipid bilayer derived from the modified host cell membrane and studded with an outer layer of virus envelope glycoproteins.
Viruses are classified according to morphology, chemical composition, and mode of replication. Pathogenesis is the process by which virus infection leads to disease. Pathogenic mechanisms include inserting of the virus at a body site (the portal of entry), replication at that site, and then spread to and multiplication within sites (target organs) where disease or shedding of virus into the environment happens. Most viral infections are subclinical, suggesting that body defenses against viruses arrest most infections before disease symptoms appear.
The survival of humans and animals, most natural selective pressures favor the authority of less virulent strains. Because these strains do not cause severe disease or death, their replication and transmission are not decreased by an incapacitated host. Mild infections can result from the absence of one or more virulence factors. For example, a virus that has all the virulence characteristics except the ability to multiply at elevated temperatures is arrested at the febrile stage of infection and causes a milder disease than its virulent counterpart.
Vaccines are made to induce the host’s immunity through the live virus vaccines which composed of viruses deficient in one or more virulence factors; they cause only in apparent infections and yet can replicate sufficiently as the agent stimulates the body's immune system to recognize the agent as a threat, destroy it, and to further recognize and destroy any of the microorganisms associated with that agent that it may encounter in the future.
1) Gelderblom HR. Structure and Classification of Viruses. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 41. Available from: https://www.ncbi.nlm.nih.gov/books/NBK8174/.
2) Baron S, Fons M, Albrecht T. Viral Pathogenesis. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 45. Available from: https://www.ncbi.nlm.nih.gov/books/NBK8149/.