Introduction to viruses

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Headshot of Joanna Breems, MD, FACP · Clinical Assistant Professor
Joanna Breems
MD, FACP · Clinical Assistant Professor
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Table of Contents
High-yield summary

Viral structure and classification 

  • All viruses contain genetic material (DNA or RNA, never both) and a protein capsid. 

  • Some viruses have an envelope derived from the host cell membrane. 

  • Classification by genetic material: DNA viruses (e.g., Herpesviridae, Adenoviridae) and RNA viruses (e.g., Orthomyxoviridae, Retroviridae). 

  • Baltimore Classification groups viruses based on their replication strategy (I–VII). 

 

Viral life cycle 

  • Steps: Attachment → Penetration → Uncoating → Replication → Assembly → Release. 

  • Lytic cycle leads to cell death; lysogenic cycle may lead to latency or transformation. 

  • Key targets for neutralizing antibodies: viral attachment and entry proteins. 

 

Viral replication strategies 

  • DNA viruses typically replicate in the nucleus using host DNA-dependent RNA polymerase. 

  • RNA viruses replicate in the cytoplasm using viral RNA-dependent RNA polymerase (RdRp). 

  • Retroviruses (e.g., HIV) use reverse transcriptase to integrate into host DNA. 

  • RdRp lacks proofreading → high mutation rate → rapid evolution and resistance. 

 

Viral pathogenesis and host response 

  • Infectivity: ability to infect; Pathogenicity: ability to cause disease; Virulence: severity of disease. 

  • Host immune response includes innate (interferons) and adaptive (T cells, antibodies). 

  • Viruses evade immunity via intracellular replication, infecting immune cells, and rapid mutation. 

  • Cytopathic effects: cell lysis, syncytia formation, inclusion bodies. 

 

Oncogenic viruses 

  • Viruses can induce cancer by integrating into host genome and altering cell cycle regulation. 

  • Examples: 

    • HPV → cervical, anal, oropharyngeal cancers (types 16, 18) 

    • EBV → Burkitt lymphoma, nasopharyngeal carcinoma 

    • HBV/HCV → hepatocellular carcinoma 

    • HTLV-1 → adult T-cell leukemia 

 

Antiviral therapy and resistance 

  • Targets: viral entry, nucleic acid synthesis, protein processing, release. 

  • Examples: 

    • Acyclovir → inhibits viral DNA polymerase (HSV, VZV) 

    • Oseltamivir → inhibits neuraminidase (Influenza) 

    • Zidovudine → reverse transcriptase inhibitor (HIV) 

  • Resistance arises via mutations in viral enzymes (e.g., thymidine kinase, reverse transcriptase). 

 

Diagnostic principles 

  • Viruses are obligate intracellular → not visible on light microscopy. 

  • Common diagnostics: PCR, antigen detection, serology (IgM/IgG), culture (limited use). 

  • Cytopathic effects in cell culture can aid diagnosis (e.g., syncytia in measles). 

Learning goals

  1. Identify the distinctive structures of a virus and describe their use in classification of viruses and their function in the typical viral life cycle
  2. Describe the factors that enable a virus to cause disease
  3. Describe the mechanism of oncogenesis for each oncogenic virus
  4. Explain the factors that contribute to the development of antiviral resistance and provide an example

Study materials

These materials are not required; they are supplementary to large-group session. They are intended as a curated guide to content focused on the learning objectives. There are both textbook and video resources for this session for students to use per their preference. For each reference, I have designated the learning goal addressed with a learning goal icon and and number.

Click the  book icons below to go to the library resources listed.

1 Chapter 6: Viruses—Basic Concepts

Familiarize yourself with terminology and variations in sections on viral components (Viral structure, Genome structure, and Capsid structure).

Figure 6-7. Virus Replication Cycle: A general scheme of the six discrete steps of virus replication cycle, including attachment, penetration, uncoating, synthetic phase (transcription, translation and replication), assembly, and release. See text for detailed description of each step.
  • 3 Section on Viral transformation: Note types of viruses and mechanisms associated with oncogenic transformation.
  • 2 Section on Host Defenses, Adaptive Immune Responses, and Virus-Induced Immunopathology and Immunosuppression: Focus specifically on the section on virulence and cytopathogenicity.
  • Sections on General Considerations and Antiviral Resistance: Focus on the discussion of what factors increase likelihood of resistant mutations.
  • Section on Key Conclusions: Read key conclusions in light of basic viral structure and function. (specific antiviral agents will be covered later in the course).
Figure 8-1. General scheme of antiviral action. Good graphic representation of the various targets of antiviral therapy. This is included as an introduction to anti-viral therapy. Specific agents will be covered with specific pathogens throughout the course.

Additional resources

Figure 29-5. Example of viral growth cycles. A: The growth cycle of a nonenveloped, double-stranded DNA virus. In this example multiple steps in the replication cycle take place in the nucleus. (1) After penetrating the host cell, viral DNA is uncoated and enters the nucleus. (2) Viral genes are transcribed. (3) The mRNAs are translated in the cytoplasm. Newly synthesized proteins enter the nucleus. (4) Viral DNA is replicated in the nucleus, sometimes with the help of newly synthesized viral replication proteins. (5) Viral DNA and viral structural proteins assemble in the nucleus to produce new progeny virions. (6) On rare occasions, viral DNA may be incorporated into cellular DNA as a side effect of infection. B: The growth cycle of a positive-sense, single-stranded RNA virus. In this example, the replication cycle occurs in the cytoplasm. (1) The virus enters the cell and the viral RNA genome is uncoated. (2) As a positive-sense, single-stranded genome, the RNA is directly translated, producing viral proteins. (3) A negative-sense RNA copy of the positive template is synthesized. (4) It is used to produce many positive-sense copies. (5) The newly synthesized positive-sense RNA molecules are assembled with viral structural proteins to produce new progeny virions. (Reproduced with permission from Talaro KP: Foundations in Microbiology: Basic Principles, 6th ed. McGraw-Hill, 2008. © The McGraw-Hill Companies, Inc.) Carroll KC, Hobden JA, Miller S, et al. Jawetz, Melnick, & Adelbergs Medical Microbiology. 27th ed. New york: McGraw-Hill Education; 2016.
Figure 31-1. Classification scheme for DNA Viruses. Suggested as a reference. Memorization the classification of all viral types is less important than applying conceptual knowledge of difference between DNA and RNA viruses. (medical exams have less viral classification now than previously).
Figure 31-2. Classification scheme for RNA Viruses. Suggested as a reference. Memorization the classification of all viral types is less important than applying conceptual knowledge of difference between DNA and RNA viruses. (medical exams have less viral classification now than previously).