Virology Calculators

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Virology is the branch of microbiology that studies viruses — obligate intracellular parasites consisting of nucleic acid (DNA or RNA) enclosed in a protein coat (capsid) and sometimes a lipid envelope. Viruses cannot independently replicate and require a host cell's machinery. The Baltimore classification system organizes viruses by genome type and replication strategy (Classes I–VII). Viruses cause diseases ranging from the common cold (rhinovirus) to HIV/AIDS, influenza, hepatitis, SARS-CoV-2, and Ebola. Antiviral drugs and vaccines are the primary tools for controlling viral diseases.

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Viral Structure

  • Capsid: Protein shell surrounding nucleic acid; icosahedral or helical symmetry; provides structural protection
  • Nucleic acid: DNA or RNA (single- or double-stranded); positive- or negative-sense (for RNA viruses); size 3–300 kb
  • Envelope: Some viruses have a host-derived lipid bilayer surrounding the capsid; contains viral glycoproteins for cell entry; sensitive to detergents and drying. Non-enveloped viruses are more environmentally stable
  • Receptor-binding proteins: Hemagglutinin (influenza), spike protein (SARS-CoV-2), gp120 (HIV) — determine host cell tropism

Baltimore Classification

  • Class I: dsDNA (adenovirus, herpes)
  • Class II: ssDNA (parvovirus)
  • Class III: dsRNA (reovirus, rotavirus)
  • Class IV: (+)ssRNA (SARS-CoV-2, polio, dengue)
  • Class V: (−)ssRNA (influenza, measles, Ebola)
  • Class VI: ssRNA-RT (HIV, HTLV)
  • Class VII: dsDNA-RT (hepatitis B)

Viral Replication Cycle

Attachment → Entry (endocytosis or membrane fusion) → Uncoating → Replication (nucleus or cytoplasm) → Assembly → Release (budding or lysis).

Antiviral Drugs

Target viral-specific steps: nucleoside analogs (acyclovir, remdesivir) inhibit viral polymerase; protease inhibitors (HIV); neuraminidase inhibitors (oseltamivir/Tamiflu); fusion inhibitors; reverse transcriptase inhibitors.

Glossary

Virus
An obligate intracellular parasite consisting of nucleic acid (DNA or RNA) in a protein capsid, sometimes with a lipid envelope; cannot independently replicate; infects all life forms.
Baltimore Classification
A system organizing viruses by genome type and replication strategy (Classes I–VII); Class IV (+)ssRNA includes SARS-CoV-2; Class VI ssRNA-RT includes HIV; guides antiviral drug target selection.
Capsid
The protein coat surrounding viral nucleic acid; icosahedral or helical symmetry; provides protection and mediates cell entry; the target of neutralizing antibodies in many viral infections.

Frequently Asked Questions

Viruses are obligate intracellular parasites — they cannot independently metabolize, grow, or replicate. They consist of nucleic acid (DNA or RNA) in a protein coat (capsid), sometimes with a lipid envelope. Bacteria: free-living, single-celled prokaryotes with their own metabolism, ribosomes, and ability to reproduce independently. Key differences: size — bacteria 0.5–5 μm; viruses 20–300 nm. Structure — bacteria have cell walls, membranes, ribosomes; viruses have only capsid and nucleic acid. Reproduction — bacteria divide by binary fission; viruses replicate only inside host cells using host machinery. Sensitivity — most antibiotics target bacteria (cell wall, ribosomes, DNA gyrase); antivirals target virus-specific enzymes or entry mechanisms; antibiotics have no effect on viral infections.

Baltimore classification (1971, Nobel Prize 1975) organizes viruses by their genome type and replication strategy — how they produce mRNA (the universal intermediate). Seven classes: I: dsDNA (adenovirus, herpesvirus, poxvirus) — use host DNA-dependent RNA polymerase. II: ssDNA (parvovirus) — convert to dsDNA first. III: dsRNA (rotavirus) — encode their own RNA-dependent RNA polymerase. IV: (+)ssRNA (SARS-CoV-2, poliovirus, dengue, hepatitis C) — genome acts directly as mRNA; RNA-dependent RNA polymerase needed for replication. V: (−)ssRNA (influenza, measles, Ebola) — complementary to mRNA; must encode RNA polymerase in virion. VI: ssRNA with reverse transcriptase (HIV, HTLV) — convert RNA to DNA that integrates into host genome. VII: dsDNA with reverse transcriptase (hepatitis B) — partial dsDNA; use RT for replication.

Antiviral drugs target virus-specific processes while sparing host cells. Main targets: Entry: fusion inhibitors (enfuvirtide for HIV); receptor blockers. Uncoating: amantadine (influenza M2 channel). Replication: nucleoside analogs (acyclovir for herpes, remdesivir for SARS-CoV-2) are incorporated into viral DNA/RNA and terminate chain elongation; protease inhibitors (HIV, HCV) block polyprotein cleavage; neuraminidase inhibitors (oseltamivir) prevent influenza release from cells. Reverse transcriptase inhibitors (HIV). Why harder than antibiotics: viruses use host cell machinery (ribosomes, many polymerases, membrane lipids) — blocking these also harms the host; viral enzymes (polymerases, proteases) must be sufficiently different from host enzymes; rapidly evolving viruses develop resistance quickly.

Vaccines stimulate adaptive immunity (antibodies + T cells) to produce memory cells that can rapidly respond upon subsequent exposure. Types and mechanisms: Live attenuated (measles, MMR): weakened virus stimulates strong, long-lasting immune response; mimics natural infection. Inactivated/killed (influenza, poliovirus IPV): killed virus; safe but weaker response; requires boosters. Subunit (hepatitis B, HPV): purified viral proteins; safe; requires adjuvants. mRNA vaccines (SARS-CoV-2): cells make spike protein → immune response; no DNA integration; rapidly designed and manufactured. Viral vector (adenovirus-based): delivers antigen gene using a harmless viral vector. Effective vaccines require the antigen to be immunodominant, conserved across strains, and accessible to neutralizing antibodies — the spike protein of SARS-CoV-2 was an ideal target for vaccine development.