Introduction to Virology

Study Notes

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.

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.

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.

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.

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.

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.

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.

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.

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.