Viruses: Structure and Diseases
Deep FCI AG-III Technical Zoology lesson on viral structure, types, replication, bacteriophages, plant and animal viral diseases, food storage relevance and conceptual clarifications.
Biological and Public Health Importance of Viruses
Viruses are infectious particles that behave like living organisms only inside a host cell. They are central to exam questions because they sit at the boundary between living and non-living systems.
For FCI AG-III Technical, focus on:
- Viral structure and genetic material.
- Difference from bacteria and protozoa.
- Human, animal and plant diseases.
- Bacteriophages.
- Food and storage relevance through contamination, hygiene and animal-origin foods.
Basic Nature of Viruses
| Feature | Virus |
|---|---|
| Cell status | Acellular; not a true cell |
| Size | Smaller than bacteria; generally nanometre scale |
| Genetic material | Either DNA or RNA, never both as the main genome |
| Protein synthesis machinery | Absent |
| Ribosomes | Absent |
| Metabolism | No independent metabolism |
| Multiplication | Only inside living host cell |
| Treatment | Antibiotics do not work |
Viruses are called obligate intracellular parasites because they must enter a living host cell to multiply.
conceptual confusion: A virus is not a prokaryote. Bacteria are prokaryotic cells; viruses are acellular infectious particles.
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Biological and Public Health Importance of Viruses
Viruses are infectious particles that behave like living organisms only inside a host cell. They are central to exam questions because they sit at the boundary between living and non-living systems.
For FCI AG-III Technical, focus on:
- Viral structure and genetic material.
- Difference from bacteria and protozoa.
- Human, animal and plant diseases.
- Bacteriophages.
- Food and storage relevance through contamination, hygiene and animal-origin foods.
Basic Nature of Viruses
| Feature | Virus |
|---|---|
| Cell status | Acellular; not a true cell |
| Size | Smaller than bacteria; generally nanometre scale |
| Genetic material | Either DNA or RNA, never both as the main genome |
| Protein synthesis machinery | Absent |
| Ribosomes | Absent |
| Metabolism | No independent metabolism |
| Multiplication | Only inside living host cell |
| Treatment | Antibiotics do not work |
Viruses are called obligate intracellular parasites because they must enter a living host cell to multiply.
conceptual confusion: A virus is not a prokaryote. Bacteria are prokaryotic cells; viruses are acellular infectious particles.
Living and Non-Living Characters
| Living character | Non-living character |
|---|---|
| Contains genetic material | No cellular organization |
| Mutates and evolves | No ribosomes |
| Multiplies inside host cells | No independent metabolism |
| Host specificity | Can be crystallized |
| Causes infection | Inactive outside host |
This dual nature is why viruses are often described as a link between living and non-living matter.
General Structure of a Virus
1. Genome
The viral genome may be:
- DNA or RNA.
- Single-stranded or double-stranded.
- Linear or circular.
- Segmented or non-segmented.
| Genome type | Examples |
|---|---|
| DNA virus | Herpes virus, adenovirus, poxvirus |
| RNA virus | Influenza virus, rabies virus, poliovirus, coronavirus |
| Retrovirus | HIV; RNA genome with reverse transcriptase |
2. Capsid
The genome is surrounded by a protein coat called the capsid. The capsid is built from smaller protein units called capsomeres.
Functions of capsid:
- Protects genetic material.
- Helps attach to host cell.
- Gives shape to the virus.
- Contains antigenic proteins recognized by the immune system.
3. Envelope
Some viruses have an outer lipid envelope derived from host cell membrane.
| Enveloped viruses | Non-enveloped viruses |
|---|---|
| Have lipid envelope | No lipid envelope |
| Usually sensitive to soap, detergents and drying | Usually more resistant in environment |
| Examples: influenza, HIV, coronavirus, herpes | Examples: poliovirus, adenovirus, many enteric viruses |
Practical point: Soap disrupts lipid envelopes. This is why handwashing is especially effective against many enveloped viruses.
Viral Shapes
| Shape | Description | Example |
|---|---|---|
| Helical | Capsid arranged as helix | Tobacco mosaic virus |
| Icosahedral | Many triangular faces; roughly spherical | Adenovirus |
| Enveloped | Capsid plus lipid envelope | Influenza virus |
| Complex | Head-tail structure | Bacteriophage |
Bacteriophage Structure
Bacteriophages are viruses that infect bacteria. The T4 phage is a classic example.
| Part | Function |
|---|---|
| Head / capsid | Contains viral DNA |
| Tail sheath | Contracts to inject DNA |
| Tail fibre | Attachment to bacterial cell |
| Base plate | Anchoring during infection |
Lytic Cycle
- Attachment to bacterial cell.
- Injection of viral nucleic acid.
- Synthesis of viral parts using host machinery.
- Assembly of new phage particles.
- Lysis of host cell and release.
Lysogenic Cycle
In lysogeny, viral DNA integrates into bacterial DNA as a prophage and replicates with the host without immediate lysis.
| Lytic cycle | Lysogenic cycle |
|---|---|
| Host cell is destroyed quickly | Host cell survives initially |
| Rapid release of viruses | Viral genome remains as prophage |
| Acute infection pattern | Dormant or latent phase |
| Ends in lysis | Can later shift to lytic cycle |
conceptual confusion: In bacteriophage infection, usually only nucleic acid enters the bacterial cell. The protein coat remains outside in classic phage examples.
Viral Replication: General Steps
| Step | What happens |
|---|---|
| Attachment | Virus binds specific receptor on host cell |
| Penetration | Virus or viral genome enters cell |
| Uncoating | Genome is released from capsid |
| Biosynthesis | Viral nucleic acid and proteins are made |
| Assembly | New viral particles are formed |
| Release | Viruses exit by lysis or budding |
Host specificity depends on receptor compatibility. This is why a virus may infect one tissue or species but not another.
Important Human Viral Diseases
| Disease | Virus | Key clue / transmission |
|---|---|---|
| Common cold | Rhinoviruses and others | Respiratory droplets |
| Influenza | Influenza virus | Fever, body ache, respiratory spread |
| COVID-19 | SARS-CoV-2 | Respiratory infection |
| Measles | Measles virus | Fever, rash, Koplik spots |
| Mumps | Mumps virus | Swelling of parotid gland |
| Chickenpox | Varicella-zoster virus | Vesicular rash |
| Polio | Poliovirus | Faeco-oral route; paralysis in severe cases |
| Rabies | Rabies virus | Bite of infected animal; fatal encephalitis |
| Hepatitis A | HAV | Faeco-oral route |
| Hepatitis B | HBV | Blood, sexual contact, mother to child |
| AIDS | HIV | Attacks immune system, especially CD4 cells |
| Dengue | Dengue virus | Aedes mosquito |
| Chikungunya | Chikungunya virus | Aedes mosquito |
| Japanese encephalitis | JE virus | Mosquito-borne |
conceptual confusion: Dengue and chikungunya are viral diseases transmitted by Aedes mosquitoes. Malaria is not viral; it is protozoan and transmitted by female Anopheles.
Important Animal Viral Diseases
| Disease | Main host / importance | Key point |
|---|---|---|
| Foot and mouth disease | Cattle, buffalo, sheep, goats, pigs | Highly contagious; affects livestock productivity |
| Rinderpest | Cattle | Historically important; declared eradicated globally |
| Rabies | Mammals | Zoonotic; transmitted through bite/saliva |
| Newcastle disease | Poultry | Viral disease of birds |
| Avian influenza | Birds; zoonotic risk in some strains | Important for poultry hygiene |
| Classical swine fever | Pigs | Viral livestock disease |
Animal viral diseases matter in food systems because livestock health affects milk, meat, eggs, trade and biosecurity.
Important Plant Viral Diseases
| Disease | Plant | Key symptom / clue |
|---|---|---|
| Tobacco mosaic | Tobacco | Mosaic pattern on leaves |
| Yellow vein mosaic | Bhendi / okra | Yellowing of veins |
| Leaf curl | Tomato, chilli, cotton and others | Curling and stunting |
| Bunchy top | Banana | Bunched leaves at top |
| Tungro | Rice | Stunting and yellow-orange leaf discoloration |
| Mosaic disease | Many crops | Patchy green and yellow areas |
Plant viruses are commonly spread by insect vectors such as aphids, whiteflies, leafhoppers and thrips.
| Vector | Common viral disease association |
|---|---|
| Aphid | Many mosaic viruses |
| Whitefly | Leaf curl and yellow vein mosaic |
| Leafhopper | Rice tungro |
| Thrips | Some tospoviruses |
FCI link: Viral diseases reduce crop yield and quality before procurement. They are not usually the main cause of dry grain spoilage in storage, but they affect production and marketable quality.
Viruses and Food Safety
Viruses do not multiply in dry stored grains the way bacteria and fungi can. However, they are important in food safety because contaminated food and water can transmit viral diseases.
| Virus / disease | Food or water relevance |
|---|---|
| Hepatitis A | Contaminated water, raw foods handled unhygienically |
| Norovirus | Food handlers, contaminated surfaces, outbreaks |
| Poliovirus | Faeco-oral contamination |
| Rotavirus | Contaminated hands, water, surfaces; diarrhoea in children |
Important Distinction
| Point | Bacteria | Viruses |
|---|---|---|
| Can multiply in food? | Many can multiply if conditions are favourable | Generally need living host cells, so do not multiply in ordinary food |
| Toxin production in food | Some produce toxins | Usually disease is by infection, not food toxin production |
| Antibiotics | May work | Do not work |
| Control | Cooking, hygiene, temperature, moisture | Hygiene, safe water, vaccination, avoiding contamination |
Prevention and Control
| Method | Role |
|---|---|
| Vaccination | Prevents many viral diseases such as measles, polio, rabies after exposure, hepatitis B |
| Handwashing | Removes and inactivates many viruses |
| Safe water | Prevents faeco-oral viral diseases |
| Vector control | Reduces dengue, chikungunya and JE |
| Isolation | Reduces respiratory spread |
| Food-handler hygiene | Prevents foodborne viral outbreaks |
| Proper cooking | Inactivates many viruses, though hygiene before cooking still matters |
conceptual confusion: Antibiotics do not cure viral infections. Antiviral drugs and vaccines are different from antibiotics.
Viruses vs Viroids vs Prions
| Agent | Composition | Disease relevance |
|---|---|---|
| Virus | Nucleic acid plus protein coat; some have envelope | Many plant, animal and human diseases |
| Viroid | Small circular RNA without protein coat | Plant diseases |
| Prion | Misfolded protein, no nucleic acid | Degenerative nervous system diseases |
Comparative Summary: Bacteria and Viruses
| Feature | Bacteria | Viruses |
|---|---|---|
| Organization | Cellular | Acellular |
| Nucleus | No true nucleus | No nucleus |
| Cell wall | Often peptidoglycan | No cell wall; capsid present |
| Ribosome | 70S present | Absent |
| Growth medium | Can grow on artificial media if suitable | Need living cells |
| Reproduction | Binary fission | Assembly inside host cell |
| Antibiotic effect | Often affected | Not affected |
| Example disease | Cholera, typhoid, TB | Rabies, influenza, polio |
Common Conceptual Confusions
| Trap | Correct point |
|---|---|
| Viruses are unicellular | False. They are acellular |
| Viruses have both DNA and RNA as genome | False. A virus has either DNA or RNA as its genome |
| Antibiotics kill viruses | False |
| Bacteriophage infects humans directly | Bacteriophage infects bacteria |
| Virus can multiply in stored dry grain | Viruses require living host cells; they do not multiply like storage fungi or bacteria |
| Dengue is bacterial | False. Dengue is viral |
| Malaria is viral | False. Malaria is protozoan |
| TMV is animal virus | False. Tobacco mosaic virus infects plants |
Summary Cheat Sheet
| Topic | Key points |
|---|---|
| Nature | Acellular, obligate intracellular parasite |
| Structure | Genome plus capsid; envelope present in some |
| Genetic material | DNA or RNA, not both as main genome |
| Ribosome | Absent |
| Replication | Attachment, penetration, uncoating, biosynthesis, assembly, release |
| Bacteriophage | Virus infecting bacteria; T4 has head, tail, tail fibres |
| Lytic cycle | Host destroyed and viruses released |
| Lysogenic cycle | Viral DNA remains as prophage |
| Human diseases | Influenza, polio, rabies, dengue, hepatitis, AIDS, COVID-19 |
| Plant diseases | TMV, leaf curl, yellow vein mosaic, tungro |
| Control | Vaccination, hygiene, safe water, vector control |
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