Viruses are submicroscopic infectious agents that can multiply only inside living host cells (animals, plants, bacteria, or fungi).
They are non-cellular, consisting mainly of nucleic acid and protein, and lack independent metabolic machinery.
Although extremely small (20–300 nm), viruses are among the most abundant biological entities on Earth and play key roles in disease, genetics, and molecular biology.
Thank you for reading. Don't forget to subscribe & share!
2. History of Viruses
The discovery and study of viruses marked a major milestone in microbiology.
Below is a chronological summary of the historical development of virology:
| Year | Scientist(s) | Discovery / Contribution |
| 1886 | Adolf Mayer (Germany) | Demonstrated that the juice from infected tobacco plants could transmit tobacco mosaic disease to healthy plants, suggesting a contagious agent. |
| 1892 | Dmitri Ivanovsky (Russia) | Passed infected sap through a porcelain filter (Chamberland filter) that trapped bacteria — the filtrate still caused disease. Proposed an “infectious agent smaller than bacteria.” |
| 1898 | Martinus Beijerinck (Netherlands) | Confirmed Ivanovsky’s results and called the agent Contagium vivum fluidum (contagious living fluid) — first recognition of a virus (Tobacco Mosaic Virus, TMV). |
| 1898 | Friedrich Loeffler & Paul Frosch | Discovered the first animal virus — Foot-and-Mouth Disease Virus (FMDV). |
| 1915 | Frederick Twort | Discovered viruses that infect bacteria — bacteriophages. |
| 1917 | Félix d’Hérelle | Independently discovered bacteriophages and coined the term. |
| 1935 | Wendell M. Stanley | Crystallized the Tobacco Mosaic Virus (TMV) — proving that viruses can be purified and are mainly composed of protein and nucleic acid. |
| 1940s–1950s | Electron microscopy | Enabled direct visualization of viruses for the first time. |
| 1952 | Hershey & Chase | Demonstrated that DNA is the genetic material in bacteriophage (phage T2) infection. |
| 1950–Present | Various scientists | Discovery of animal, plant, and human viruses; understanding of viral genetics, replication, and pathogenesis. |
3. General Characteristics of Viruses
- Acellular: Lack cellular structure; cannot perform metabolism independently.
- Obligate intracellular parasites: Reproduce only inside living host cells.
- Genetic Material: Contain either DNA or RNA, never both.
- Non-living outside host: Inert and crystallizable when outside cells.
- Ultramicroscopic: Size ranges from 20 nm (Parvoviruses) to 400 nm (Poxviruses).
- Specificity: Highly specific for host species and cell type (tropism).
- Reproduction: Through replication of nucleic acid and synthesis of viral proteins using host machinery.
- No ribosomes, enzymes, or organelles for energy production.
4. Structure of Viruses
Despite great diversity, most viruses share a basic structural plan consisting of:
- Nucleic Acid Core (Genome)
- Protein Coat (Capsid)
- Envelope (in some viruses)
- Surface Projections (Spikes) — optional structures for attachment
A. Nucleic Acid (Viral Genome)
- The genetic material may be:
- DNA or RNA, but never both.
- Single-stranded (ss) or Double-stranded (ds).
- Linear or Circular, segmented or continuous.
| Type of Genome | Example |
| dsDNA | Adenovirus, Herpesvirus, Poxvirus |
| ssDNA | Parvovirus |
| dsRNA | Reovirus |
| ssRNA (+ sense)** | Poliovirus, Dengue virus |
| ssRNA (− sense)** | Influenza virus, Rabies virus |
| Retrovirus (ssRNA → DNA)** | HIV |
(+ sense RNA acts like mRNA; (− sense RNA must be converted to + sense before translation.)
B. Capsid
- The protein shell that encloses and protects the nucleic acid.
- Composed of capsomeres, which are repeating protein subunits.
- Functions:
- Protects viral genome from degradation.
- Determines the shape of the virus.
- Plays a role in host cell recognition and attachment.
- Facilitates transfer of nucleic acid into the host cell.
Capsid Symmetry Types
| Symmetry Type | Description | Examples |
| Helical | Rod-like; nucleic acid coiled in a spiral inside capsid | Tobacco Mosaic Virus, Influenza virus |
| Icosahedral | 20 triangular faces; spherical in appearance | Adenovirus, Herpesvirus, Poliovirus |
| Complex | Neither purely helical nor icosahedral; may have tail or other structures | Bacteriophage T4, Poxvirus |
C. Envelope
- Some viruses have a lipoprotein envelope derived from the host cell membrane during viral release.
- The envelope contains glycoprotein spikes essential for host recognition and attachment.
- Enveloped viruses are sensitive to heat, desiccation, and detergents, while non-enveloped (naked) viruses are more resistant.
| Enveloped Viruses | Non-Enveloped (Naked) Viruses |
| Influenza virus, HIV, Herpesvirus | Poliovirus, Adenovirus, Norovirus |
D. Surface Projections (Spikes or Peplomers)
- Found on the surface of many enveloped viruses.
- Composed of glycoproteins that act as antigens and help the virus:
- Attach to host receptors,
- Enter host cells, and
- Elicit immune responses.
Examples:
- Influenza virus: Hemagglutinin (HA) and Neuraminidase (NA) spikes.
- Coronavirus: Spike (S) protein responsible for host cell binding.
5. Composition of Viruses
Viruses are mainly composed of nucleic acid and protein, with some having lipids and carbohydrates.
| Component | Description |
| Nucleic Acid | Either DNA or RNA; carries genetic information required for replication and synthesis of viral components. |
| Proteins | Make up the capsid; protect genome and form structural and enzymatic components. |
| Lipids | Present in the envelope; derived from host cell membrane phospholipids. |
| Carbohydrates | Associated with glycoproteins on viral envelopes and spikes. |
| Enzymes (in some viruses) | Reverse transcriptase (Retroviruses), RNA polymerase (Influenza virus), proteases, integrases. |
Chemical Composition (Typical Ranges)
| Component | Percentage (approx.) |
| Protein | 50–90% |
| Nucleic Acid | 5–40% |
| Lipid | 0–20% (only in enveloped viruses) |
| Carbohydrate | 1–5% (in glycoproteins) |
6. Examples of Viral Structures
| Virus | Genome Type | Structure | Envelope |
| Tobacco Mosaic Virus | ssRNA | Helical | None |
| Adenovirus | dsDNA | Icosahedral | None |
| Influenza Virus | ssRNA (−) | Helical | Present |
| Herpes Simplex Virus | dsDNA | Icosahedral | Present |
| Bacteriophage T4 | dsDNA | Complex | None |
| HIV | ssRNA (+, retrovirus) | Icosahedral | Present |
7. Significance of Viral Structure and Composition
- Host Specificity: Determined by the type of viral receptors and spikes.
- Pathogenicity: The envelope and proteins determine how the virus enters and damages host cells.
- Immune Response: Viral proteins and glycoproteins act as antigens to stimulate immunity.
- Vaccine Development: Knowledge of structure aids in designing vaccines (e.g., COVID-19 spike protein vaccines).
- Therapeutic Targets: Viral enzymes and structural proteins are targets for antiviral drugs.
