Viruses

Overview of Viruses

  • Viruses are nonliving entities that require living cells to reproduce.

  • The understanding of viruses evolved in the late 1800s with the germ theory of disease.

Early Discoveries

  • 1892: Ivanowski and Beijerinck study tobacco mosaic disease, discovering a filterable virus in the filtrate.

  • 1898: Frosh and Loeffler report hoof-and-mouth disease in animals caused by a filterable virus, establishing that viruses can be transmitted between animals.

  • 1902: Walter Reed identifies yellow fever as caused by a filterable virus, marking human viral involvement.

  • The development of the electron microscope in the 1930s allowed for visualization of viruses.

Debate on Virus Classification

  • Living Characteristics:

    • Cause fever, elicit antibody responses, reproduce, contain macromolecules, evolve/adapt.

  • Nonliving Arguments:

    • Crystallizable nature; lack of organelles; depend on host cells for reproduction (obligate intracellular parasites).

  • Most scientists classify viruses as active/inactive rather than living/dead.

Classification of Viruses

  1. Host Range: Specific spectrum of organisms the virus infects; determined by the presence of appropriate receptors.

    • Example: HIV targets T-helper cells.

  2. Size: Ranges from 10 nm to 500 nm; detected only after electron microscope development.

  3. Structure: Composed of:

    • Core of Nucleic Acid: Can be DNA (double/single stranded) or RNA (double/single stranded).

    • Capsid: Protective protein coat; can have helical, isometric, or complex shapes.

    • Envelope: Not present in all viruses; acquired from the host during replication, sometimes featuring spikes for attachment.

  4. Route of Transmission:

    • Enteric: Fecal-oral, e.g., rotavirus.

    • Respiratory: Inhaled droplets, e.g., influenza.

    • Zoonotic: Animal to human, e.g., rabies.

    • Sexually Transmitted: E.g., HIV.

  5. Life Cycle:

    • Lytic Cycle: Virus destroys host cell; used in phage therapy for antibiotic-resistant bacteria.

    • Lysogenic Cycle: Viral DNA integrates with bacterial DNA; can enable genetic engineering.

    • Animal Viruses: May destroy or integrate their genetic information with animal cells.

  6. Interaction with Animal Cells:

    • Acute Infections: Short-duration, often leading to immunity (e.g., common cold).

    • Persistent Infections: Continuous presence of virus, can be latent (e.g., varicella zoster) or chronic (e.g., hepatitis B).

Unusual Virus Groups

  • Retroviruses: RNA viruses that synthesize DNA (e.g., HIV) using reverse transcriptase.

  • Influenza: Segmented viral genome; undergoes antigenic drift and shift, requiring annual vaccination adaptations.

  • Tumor Viruses: Cause cancer by disrupting genes that control cell division (e.g., HPV, EBV, hepatitis C).