7: Viruses
Public health measures in the developed world have dramatically reduced mortality from viral epidemics. But when epidemics do occur, they can spread quickly with global air travel. In 2009, an outbreak of H1N1 influenza spread across various continents. In early 2014, cases of Ebola in Guinea led to a massive epidemic in western Africa. This included the case of an infected man who traveled to the United States, sparking fears the epidemic might spread beyond Africa.
Until the late 1930s and the advent of the electron microscope, no one had seen a virus. Yet treatments for preventing or curing viral infections were used and developed long before that. Historical records suggest that by the 17th century, and perhaps earlier, inoculation (also known as variolation) was being used to prevent the viral disease smallpox in various parts of the world. By the late 18th century, Englishman Edward Jenner was inoculating patients with cowpox to prevent smallpox, a technique he coined vaccination . 1
Today, the structure and genetics of viruses are well defined, yet new discoveries continue to reveal their complexities. In this chapter, we will learn about the structure, classification, and cultivation of viruses, and how they impact their hosts. In addition, we will learn about other infective particles such as viroids and prions.
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- 7.1: Viruses
- Viruses are generally ultramicroscopic, typically from 20 nm to 900 nm in length. Some large viruses have been found. Virions are acellular and consist of a nucleic acid, DNA or RNA, but not both, surrounded by a protein capsid. There may also be a phospholipid membrane surrounding the capsid. Viruses are obligate intracellular parasites.
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- 7.2: The Viral Life Cycle
- Many viruses target specific hosts or tissues. Some may have more than one host. Many viruses follow several stages to infect host cells. These stages include attachment, penetration, uncoating, biosynthesis, maturation, and release. Bacteriophages have a lytic or lysogenic cycle. The lytic cycle leads to the death of the host, whereas the lysogenic cycle leads to integration of phage into the host genome.
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- 7.3: Viruses
- A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.
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- 7.3.1: General Characteristics of Viruses
- 7.3.2: Size and Shapes of Viruses
- 7.3.3: Viral Structure
- 7.3.4: Classification of Viruses
- 7.3.5: Other Acellular Infectious Agents- Viroids and Prions
- 7.3.6: Animal Virus Life Cycles
- 7.3.6A: The Productive Life Cycle of Animal Viruses
- 7.3.6B: Productive Life Cycle with Possible Latency
- 7.3.6C: The Life Cycle of HIV
- 7.3.6D: Natural History of a Typical HIV Infection
- 7.3.6E: The Role of Viruses in Tumor Production
- 7.3.7: Bacteriophage Life Cycles- An Overview
- 7.3.7A: The Lytic Life Cycle of Bacteriophages
- 7.3.7B: The Lysogenic Life Cycle of Bacteriophages
- 7.3.8: Pathogenicity of Animal Viruses
- 7.3.9: Bacteriophage-Induced Alterations of Bacteria
- 7.3.10: Antiviral Agents
- 7.3.11: General Categories of Viral Infections
- 7.3.E: Viruses (Exercises)
Footnotes
- 1 S. Riedel “Edward Jenner and the History of Smallpox and Vaccination.” Baylor University Medical Center Proceedings 18, no. 1 (January 2005): 21–25.
Thumbnail: This colorized transmission electron microscopic (TEM) image revealed some of the ultrastructural morphology displayed by an Ebola virus virion. (Public Domain; Frederick A. Murphy via CDC).