Viruses

Overview of Viruses

• Definition: Microscopic infectious agents that can replicate only inside the living cells of organisms.
• Study field: Virology.
• Infection process: Viruses infect host cells to reproduce and can adapt to immune responses.
• Treatment: More difficult to treat than bacterial infections.

Structure of Viruses

• Components:
  • Capsid: Protein coat that encases the viral genome.
  • Viral genome: Consists of DNA or RNA.
  • Shapes of Viruses:
  • Helical (e.g., Tobacco Mosaic Virus)
  • Polyhedral (e.g., Adenovirus)
  • Spherical
  • Complex (e.g., Bacteriophage)
  • Size range: Typically between $0.020$ to $0.200$ micrometers.

Comparison: Viruses vs. Bacteria

• Viruses:

  • Size: $0.020$ – $0.20 \, ext{um}$
  • Composition: DNA/RNA surrounded by a protein coat.
  • Reproduction: Cannot reproduce without a living host; hijacks host cell machinery.

• Bacteria:

  • Size: $0.2$ – $2 \, ext{um}$
  • Composition: Single circular strand of DNA, ribosomes, cell membrane, and often a cell wall.
  • Reproduction: Can reproduce independently.

Viral Diseases and Outbreaks

• Notable Outbreaks:
  • H1N1 flu virus (1918), HIV (1981), West Nile virus (1996), Ebola virus (2014), SARS-CoV-2 (Covid-19, 2019).
• Symptoms of Viral Diseases:
  • Influenza Symptoms: Fever, severe muscle aches, cough, fatigue, nasal congestion, sore throat, chills, and headache.

Virus Lifecycle: Lytic vs. Lysogenic Cycle

• Lytic Cycle:

  1. Attachment: Viral proteins attach to host cell.
  2. Penetration: Virus enters the host cell.
  3. Biosynthesis: Viral parts are synthesized.
  4. Assembly: Viral parts are assembled.
  5. Release: New viruses exit the cell, often destroying it.

• Lysogenic Cycle:

  1. Attachment and injection of DNA.
  2. Phage DNA integrates into bacterial chromosome, becoming a prophage.
  3. Cell division occurs with the prophage being replicated.
  4. Lytic cycle can be initiated later by excision of the prophage.

Immune Response to Viruses

• Initial response:
  • Infected cells send signals that activate the immune system, recruiting macrophages.
  • Macrophages digest viruses and infected cells, resulting in dead white blood cells.
• Role of B cells:
  • Produce antibodies that tag viruses or infected cells for destruction.
  • Memory cells are formed for quicker future responses, reducing the likelihood of repeated infections from the same strain.
• Challenges: Viruses mutate, making previous memory responses less effective.

Vaccination and Immune Response

• Types of Vaccines:
  • Live attenuated: e.g., Measles, Rotavirus.
  • Inactivated: e.g., Inactivated Polio Virus.
  • Subunit and toxoid vaccines.
  • mRNA vaccines: Introduce synthetic mRNA to instruct cells to produce viral proteins for immune recognition (e.g., SARS-CoV-2).

Virus Transmission and Prevention

• Modes of Transmission:
  • Airborne particles, direct body fluid transfers, surfaces with dried fluids, vector organisms (e.g., mosquitoes).
• Prevention Strategies:
  • Maintain hygiene (e.g., handwashing).
  • Prevent close contact and use of masks in public settings.
  • Cover mouth/nose when sneezing or coughing.
  • Avoid contact with infected bodily fluids.