Introduction to Virology

Introduction to Virology

• Viruses are obligate intracellular parasites that can only replicate inside a living cell.
• The discovery of viruses involved bacteria-free filtrates that still caused disease.
• Three main hypotheses explain the origin of viruses:
  • Regressive hypothesis: Viruses are remnants of cellular organisms.
  • Cellular origin hypothesis: Viruses arose from genetic materials that acquired the ability to move between cells.
  • Co-evolution hypothesis: Viruses predated or co-evolved with their current cellular host.

Characteristics of Viruses

• Viruses contain a single type of nucleic acid, either RNA or DNA, but never both.
• Viruses have a protein coat that surrounds the nucleic acid.
• Viruses are small, multiply inside living cells using the host's synthetic machinery, and are totally dependent on living cells for existence.
• Release and maturation occur through lysis or budding.

Viral Replication

• Attachment or adsorption: The first step of viral replication, involving receptor interaction between the virus and host cell.
• Penetration: The virus enters the host cell through phagocytosis, viropexis, endocytosis, membrane fusion, or translocation.
• Uncoating: The viral capsid is separated by lysosomal enzymes to release the nucleic acid into the cytoplasm.
• Biosynthesis: The viral components are synthesized, including nucleic acid, capsid protein, enzymes for replication, and regulatory proteins.

Viral Replication (continued)

• For DNA viruses, replication occurs in the nucleus (except for poxviruses), and mRNA transcription and protein translation take place in the cytoplasm.
• For RNA viruses, replication occurs in the cytoplasm (except for retroviruses and orthomyxoviruses), and the RNA genome is either positive or negative sense.

Pathogenesis of Viruses

• Viral infections can be categorized into inapparent, apparent, and latent or persistent infections.
• Apparent infections can be acute, sub-acute, or chronic.
• Viruses can cause cell death through various mechanisms, including inhibition of host cell DNA, protein synthesis, fusion of cells, disruption of cell cytoskeleton, immune-mediated lysis, and induction of apoptosis.

Viral Pathogenesis at the Cell Level

• Three types of infection can be produced at the cellular level by a virus:
  • Failed or abortive infection
  • Infection leading to cell death (cytocidal or lytic infection)
  • Infection without cell death
• Viruses can induce morphological changes in the host cell, including damage to the host cell chromosome, formation of inclusion bodies, and changes in staining properties.

Host Response to Viral Infections

• The human body exerts two types of resistance to viruses: non-specific (innate) and specific immune response.
• Non-specific immune response includes phagocytosis, interferon, nutrition, age, body temperature, hormones, and stress.
• Specific immune response involves antibody-mediated immunity and cell-mediated immunity.

Host Response to Viral Infections (continued)

• Antibodies protect against viral infection, including IgG, IgA, and IgM, which work through neutralizing processes.
• Cell-mediated immune response is carried out by T-cells, which recognize viral-specific receptors and secrete lymphokines.
• Other cells involved in the immune response include NK cells, killer cells, and macrophages.

Prevention of Viral Infections

• Immunoprophylaxis is used to prevent viral diseases using vaccines and antisera or immunoglobulins.
• Vaccines can be live or killed/inactivated, and antisera or immunoglobulins can be heterologous or homologous.