Virus Calculators

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A virus is an obligate intracellular parasite — a microscopic infectious agent that can only replicate inside the living cells of a host organism. Viruses are not considered cells: they lack ribosomes, cannot produce their own ATP, and have no independent metabolism. A viral particle (virion) consists minimally of a nucleic acid genome (DNA or RNA) enclosed in a protein capsid, sometimes surrounded by a lipid envelope. Viruses infect every form of life — animals, plants, bacteria (bacteriophages), and archaea. They cause diseases from the common cold to HIV, COVID-19, hepatitis, influenza, Ebola, and rabies.

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Virus Structure

  • Genome: DNA (single or double-stranded) or RNA (single or double-stranded, + or − sense); one or multiple segments; circular or linear
  • Capsid: Protein shell composed of capsomers; icosahedral or helical symmetry; protects genome and mediates initial host cell interaction
  • Envelope (some viruses): Lipid bilayer derived from the host cell membrane during budding; embedded with viral glycoproteins (e.g., SARS-CoV-2 spike protein, influenza hemagglutinin) for receptor binding; enveloped viruses are sensitive to lipid solvents (soap, alcohol)

Viral Replication Cycle

Attachment → Penetration/Entry → Uncoating → Replication → Assembly → Release (lysis or budding). Each step is a potential antiviral drug target.

Baltimore Classification

  • I: dsDNA (herpesvirus, adenovirus)
  • II: ssDNA (parvovirus)
  • III: dsRNA (rotavirus)
  • IV: +ssRNA (SARS-CoV-2, polio, dengue)
  • V: −ssRNA (influenza, measles, Ebola)
  • VI: +ssRNA retrovirus (HIV)
  • VII: dsDNA with RNA intermediate (hepatitis B)

Host Range

Receptor specificity determines host range: SARS-CoV-2 uses ACE2 (humans, cats, ferrets); influenza uses sialic acid linkages (α2-6 in humans, α2-3 in birds). Zoonotic viruses cross species barriers when receptor compatibility exists.

Glossary

Virus
An obligate intracellular parasite: nucleic acid genome + protein capsid ± lipid envelope; replicates only in host cells using host ribosomes; causes diseases from colds to HIV and Ebola.
Capsid
The protein shell of a virus surrounding its genome; made of capsomers; icosahedral or helical symmetry; protects genome and mediates initial host cell attachment.
Enveloped Virus
A virus with a lipid bilayer (derived from host membrane) surrounding the capsid; contains viral glycoproteins for receptor binding; sensitive to soap and alcohol; examples: HIV, SARS-CoV-2, influenza.

Frequently Asked Questions

A virus is an obligate intracellular parasite that replicates only inside living host cells. Key differences from bacteria: Size: viruses 20–300 nm; bacteria 1–10 μm (bacteria are 10–1000× larger). Structure: viruses = nucleic acid + protein capsid ± lipid envelope; bacteria = cell membrane, cell wall, ribosomes, and complete metabolic machinery. Replication: viruses cannot replicate without hijacking host cell machinery (ribosomes, energy, building blocks); bacteria replicate independently. Treatment: viral infections treated with antivirals (oseltamivir, remdesivir, antiretrovirals); bacterial infections treated with antibiotics. Antibiotic resistance: antibiotic-resistant bacteria are a major clinical problem; but antibiotics have no activity against viruses. Misuse of antibiotics for viral infections (colds, flu) contributes to resistance without benefit.

Viral replication cycle: (1) Attachment: viral surface proteins bind specific receptors on host cell surface (determines host range and tissue tropism). (2) Entry/penetration: direct membrane fusion (enveloped viruses at plasma membrane or after endosome acidification); receptor-mediated endocytosis; injection (bacteriophages). (3) Uncoating: genome released from capsid into cytoplasm or nucleus. (4) Replication: viral genome transcribed and translated using host ribosomes; new viral genomes produced by viral polymerase. (5) Assembly: new capsid proteins encapsidate progeny genomes. (6) Release: lysis (non-enveloped viruses burst the cell); budding (enveloped viruses acquire envelope from host membrane while exiting — host cell may survive).

Enveloped viruses: lipid bilayer derived from host cell membrane surrounds the capsid; membrane contains viral glycoproteins (spike proteins, hemagglutinin) for receptor binding. Properties: sensitive to lipid solvents (soap, alcohol, detergents destroy the envelope → inactivates the virus); generally less resistant to environmental conditions; enter cells by membrane fusion. Examples: HIV, SARS-CoV-2, influenza, herpes, Ebola, rabies. Non-enveloped (naked) viruses: capsid protein is the outermost layer; no lipid membrane. Properties: more resistant to desiccation, pH extremes, and disinfectants (harder to kill); often transmitted via fecal-oral route (survive in gut environment). Examples: poliovirus, adenovirus, rotavirus, norovirus, HPV.

Mutation rate depends primarily on genome type and replication fidelity: RNA viruses (HIV, influenza, SARS-CoV-2): use RNA-dependent RNA polymerase (RdRp) or reverse transcriptase that LACK proofreading activity → error rate 10⁻⁴–10⁻⁵ per base per replication → high mutation rate → rapid evolution → drug resistance, immune evasion, new variants. Mutation rates: ~10⁻³–10⁻⁴ substitutions/site/year for RNA viruses. DNA viruses (herpesviruses, adenoviruses): use DNA polymerases with proofreading activity → error rate 10⁻⁷–10⁻⁹ per base → slower mutation rate → more stable genomes → less likely to rapidly evolve antiviral resistance or immune evasion. This is why influenza vaccines must be reformulated annually (RNA virus, fast evolution) while hepatitis B vaccines work for decades (DNA virus, slow evolution).