Virus

Viruses are microscopic agents of infection, capable of infecting living organisms from animals and plants to bacteria and archaea. They consist of genetic material, either DNA or RNA, encased in a protein coat. Unlike living cells, viruses cannot reproduce on their own. They hijack the cellular machinery of their host to replicate, often causing diseases. Despite their simplicity, viruses exhibit remarkable diversity and adaptability, posing significant challenges and opportunities in the fields of medicine and biotechnology. Understanding viruses is crucial for developing vaccines, antiviral drugs, and effective public health strategies.

Definition of Virus

A virus is a microscopic infectious agent that requires a living host cell to replicate. It consists of genetic material, either DNA or RNA, encased in a protein coat called a capsid. Some viruses also have an outer lipid envelope. Unlike living organisms, viruses cannot carry out metabolic processes or reproduce independently. They invade host cells and use the host’s cellular machinery to produce new virus particles, often causing diseases in the process. Viruses can infect all forms of life, including animals, plants, fungi, and bacteria.

Structure of Virus

Viruses are simple yet diverse entities with unique structures that enable them to infect host cells and replicate. The basic structure of a virus includes several key components:

1. Nucleic Acid Core

• Genetic Material: The core contains the virus’s genetic material, which can be either DNA or RNA. This genetic material can be single-stranded (ss) or double-stranded (ds), and in some cases, it can be segmented into multiple pieces.
  • DNA Viruses: Examples include herpesviruses (dsDNA) and parvoviruses (ssDNA).
  • RNA Viruses: Examples include influenza viruses (segmented ssRNA) and HIV (ssRNA).

2. Capsid

• Protein Coat: Surrounding the nucleic acid core is a protein coat called the capsid. The capsid protects the genetic material and aids in the transfer of the virus between host cells.
  • Capsomeres: The capsid is made up of protein subunits called capsomeres, which assemble in a precise and repetitive pattern.
  • Shapes: Capsids can have different shapes, including helical (rod-like), icosahedral (spherical), and complex structures.

3. Envelope (Optional)

• Lipid Bilayer: Some viruses have an outer lipid envelope derived from the host cell membrane. This envelope surrounds the capsid and contains viral glycoproteins.
  • Enveloped Viruses: Examples include influenza viruses and HIV.
  • Non-enveloped Viruses: Examples include adenoviruses and picornaviruses.

4. Viral Proteins

• Glycoproteins: Embedded in the lipid envelope (if present) are glycoproteins that play crucial roles in the virus’s ability to infect host cells. These proteins help the virus attach to and enter the host cell.
• Enzymes: Some viruses carry enzymes within their capsid that are essential for their replication. For example, retroviruses like HIV carry reverse transcriptase, which converts their RNA into DNA.

5. Additional Structures

• Matrix Proteins: Found in enveloped viruses, these proteins lie between the envelope and the capsid, providing structural support and helping with virus assembly.
• Tegument: In herpesviruses, the tegument is a layer between the capsid and the envelope that contains proteins important for viral replication and evasion of the host immune response.

Structural Examples

Helical Virus

• Example: Tobacco mosaic virus (TMV)
• Structure: Rod-shaped capsid with RNA genome. Capsomeres arranged helically around the RNA.

Icosahedral Virus

• Example: Adenovirus
• Structure: Spherical capsid with 20 triangular faces. Contains dsDNA genome.

Enveloped Virus

• Example: Influenza virus
• Structure: Helical nucleocapsid surrounded by a lipid envelope with surface glycoproteins (hemagglutinin and neuraminidase). Contains segmented ssRNA genome.

Complex Virus

• Example: Bacteriophage T4
• Structure: Icosahedral head containing dsDNA, helical tail, and tail fibers used to inject genetic material into bacterial cells.

Origins of Viruses

Viruses are unique infectious agents that require a host cell to replicate. Understanding their origins helps us grasp their role in the evolution of life and their impact on ecosystems and human health.

Scientists propose several hypotheses to explain the origins of viruses:

1. The Progressive (Escape) Hypothesis
   • Suggests viruses originated from genetic material that escaped from cells.
   • These escaped pieces of DNA or RNA acquired the ability to infect other cells.
   • Evidence: Retroviruses may have evolved from retrotransposons, genetic elements capable of moving within a genome.
2. The Regressive (Reduction) Hypothesis
   • Proposes viruses are remnants of more complex organisms that became parasitic.
   • These organisms gradually lost their cellular structures and metabolic functions.
   • Evidence: Certain large DNA viruses, like the Mimivirus, possess genes similar to those of cellular organisms.
3. The Virus-First Hypothesis
   • Asserts that viruses predate or co-evolved with the first cellular life forms.
   • Suggests viruses were self-replicating entities in the pre-cellular world.
   • Evidence: RNA viruses might be remnants of an RNA world, a hypothetical stage in the evolution of life where RNA was the primary genetic material.

Evidence Supporting Viral Origins

1. Genomic Comparisons
   • Comparisons of viral genomes with those of cellular organisms show shared genes, suggesting common ancestry or gene transfer events.
   • Example: Genes found in giant viruses, like Mimivirus and Pandoravirus, have similarities with those in eukaryotes.
2. Molecular Clock Analysis
   • Estimates the time of divergence of viral lineages by analyzing mutation rates.
   • Supports the idea that some viruses have ancient origins dating back to the early stages of life on Earth.
3. Structural Studies
   • Structural similarities between viral proteins and those of cellular organisms hint at shared evolutionary pathways.
   • Example: The capsid proteins of some viruses resemble those of certain cellular proteins, indicating a common origin.

Role of Horizontal Gene Transfer

• Horizontal Gene Transfer (HGT)
  • A significant factor in viral evolution.
  • Viruses often acquire genes from their hosts and other viruses, contributing to genetic diversity and adaptation.
  • Example: Bacteriophages (viruses that infect bacteria) frequently exchange genes with their bacterial hosts, influencing bacterial evolution.

Impact of Viruses on Evolution

• Drivers of Genetic Diversity
  • Viruses introduce new genes into populations through infection and horizontal gene transfer.
  • Example: Endogenous retroviruses, which are viral sequences integrated into the genomes of host organisms, have influenced the evolution of mammals.
• Agents of Natural Selection
  • Viral infections exert selective pressure on host populations, leading to the evolution of resistance mechanisms.
  • Example: The CRISPR-Cas system in bacteria is a defense mechanism against viral infections and has been co-opted for use in genetic engineering.

Classification of Viruses

They are classified based on various criteria, such as their genetic material, structure, replication method, and host range.

Classification by Genetic Material

1. DNA Viruses
   • Double-Stranded DNA (dsDNA) Viruses
     • Example: Adenoviruses
       • Cause respiratory infections, conjunctivitis.
     • Example: Herpesviruses
       • Cause herpes simplex, chickenpox, shingles.
   • Single-Stranded DNA (ssDNA) Viruses
     • Example: Parvoviruses
       • Cause fifth disease in children, canine parvovirus.
2. RNA Viruses
   • Double-Stranded RNA (dsRNA) Viruses
     • Example: Rotaviruses
       • Cause severe gastroenteritis in infants and young children.
   • Single-Stranded RNA (ssRNA) Viruses
     • Positive-Sense ssRNA (+ssRNA) Viruses
       • Example: Picornaviruses
         • Cause polio, hepatitis A.
       • Example: Coronaviruses
         • Cause SARS, MERS, COVID-19.
     • Negative-Sense ssRNA (-ssRNA) Viruses
       • Example: Orthomyxoviruses
         • Cause influenza.
       • Example: Filoviruses
         • Cause Ebola and Marburg hemorrhagic fevers.

Classification by Structure

1. Icosahedral Viruses
   • Have a symmetrical, icosahedral (20-sided) shape.
   • Example: Human Papillomavirus (HPV)
     • Causes warts, cervical cancer.
2. Helical Viruses
   • Have a rod-like or filamentous structure with helical symmetry.
   • Example: Tobacco Mosaic Virus (TMV)
     • Infects tobacco and other plants.
3. Complex Viruses
   • Have intricate structures that do not fit into the icosahedral or helical categories.
   • Example: Bacteriophages
     • Viruses that infect bacteria, such as T4 phage.

Classification by Host Range

1. Animal Viruses
   • Infect animal hosts, including humans.
   • Example: Rabies Virus
     • Infects mammals, causes rabies.
2. Plant Viruses
   • Infect plant hosts.
   • Example: Potato Virus Y (PVY)
     • Infects potato plants, causing mosaic disease.
3. Fungal Viruses (Mycoviruses)
   • Infect fungal hosts.
   • Example: Cryphonectria hypovirus
     • Infects the chestnut blight fungus, reduces its virulence.
4. Bacterial Viruses (Bacteriophages)
   • Infect bacterial hosts.
   • Example: Lambda Phage
     • Infects Escherichia coli bacteria.
5. Archaeal Viruses
   • Infect archaea, a domain of single-celled microorganisms.
   • Example: Sulfolobus Spindle-shaped Virus 1 (SSV1)
     • Infects Sulfolobus, an archaeon found in acidic hot springs.

Classification by Replication Method

1. Lytic Viruses
   • Cause the destruction (lysis) of the host cell during replication.
   • Example: T4 Bacteriophage
     • Infects and lyses E. coli cells.
2. Lysogenic (Temperate) Viruses
   • Integrate their genetic material into the host genome and replicate along with it without immediately lysing the host.
   • Example: Lambda Phage
     • Can integrate into the E. coli genome and enter a lysogenic cycle.
3. Retroviruses
   • Use reverse transcription to convert their RNA genome into DNA, which is then integrated into the host genome.
   • Example: Human Immunodeficiency Virus (HIV)
     • Causes AIDS.

Representative Species of Viruses

Human Viruses

1. Influenza Virus
   • Type: RNA virus
   • Family: Orthomyxoviridae
   • Diseases: Causes seasonal flu with symptoms like fever, cough, and body aches.
2. Human Immunodeficiency Virus (HIV)
   • Type: RNA virus (retrovirus)
   • Family: Retroviridae
   • Diseases: Causes AIDS, leading to immune system failure and increased susceptibility to infections and cancers.
3. Hepatitis B Virus (HBV)
   • Type: DNA virus
   • Family: Hepadnaviridae
   • Diseases: Causes hepatitis B, which can lead to chronic liver disease, cirrhosis, and liver cancer.
4. Herpes Simplex Virus (HSV)
   • Type: DNA virus
   • Family: Herpesviridae
   • Diseases: Causes oral and genital herpes, characterized by painful blisters and sores.

Animal Viruses

1. Rabies Virus
   • Type: RNA virus
   • Family: Rhabdoviridae
   • Diseases: Causes rabies, a fatal encephalitis in mammals, including humans, after being bitten by an infected animal.
2. Feline Leukemia Virus (FeLV)
   • Type: RNA virus (retrovirus)
   • Family: Retroviridae
   • Diseases: Causes leukemia and lymphoma in cats, along with immunosuppression.
3. Foot-and-Mouth Disease Virus (FMDV)
   • Type: RNA virus
   • Family: Picornaviridae
   • Diseases: Causes foot-and-mouth disease, a highly contagious viral disease affecting livestock like cattle, pigs, and sheep.

Plant Viruses

1. Tobacco Mosaic Virus (TMV)
   • Type: RNA virus
   • Family: Virgaviridae
   • Diseases: Causes mosaic disease in tobacco and other plants, leading to mottled leaves and stunted growth.
2. Potato Virus Y (PVY)
   • Type: RNA virus
   • Family: Potyviridae
   • Diseases: Affects potatoes and other plants in the nightshade family, causing leaf mottling and tuber necrosis.
3. Barley Yellow Dwarf Virus (BYDV)
   • Type: RNA virus
   • Family: Luteoviridae
   • Diseases: Affects cereals and grasses, causing yellowing and dwarfing of plants.

Bacterial Viruses (Bacteriophages)

1. T4 Bacteriophage
   • Type: DNA virus
   • Family: Myoviridae
   • Diseases: Infects E. coli bacteria, used extensively in molecular biology research.
2. Lambda Phage
   • Type: DNA virus
   • Family: Siphoviridae
   • Diseases: Infects E. coli, often used as a model organism in genetic studies.
3. PhiX174
   • Type: DNA virus
   • Family: Microviridae
   • Diseases: Infects E. coli and other bacteria, one of the first genomes to be sequenced.

Function of Virus

Infection and Disease

• Pathogenic Role
  • Viruses cause diseases in humans, animals, and plants.
  • Example: The influenza virus causes the flu, while the Human Immunodeficiency Virus (HIV) leads to AIDS.

Genetic Exchange and Evolution

• Horizontal Gene Transfer
  • Viruses facilitate the transfer of genes between different species, contributing to genetic diversity and evolution.
  • Example: Bacteriophages (viruses that infect bacteria) often carry genes from one bacterium to another, impacting bacterial evolution.
• Endogenous Retroviruses
  • Some viruses integrate their genetic material into the host genome, becoming part of the host’s genetic material.
  • Example: Endogenous retroviruses make up about 8% of the human genome and have influenced the evolution of mammals.

Regulation of Population Dynamics

• Predator-Prey Relationships
  • Viruses control the population sizes of their hosts, maintaining ecological balance.
  • Example: Bacteriophages regulate bacterial populations in various ecosystems, preventing overpopulation and promoting microbial diversity.
• Biological Pest Control
  • Certain viruses are used to control pest populations in agriculture.
  • Example: Baculoviruses infect and kill insect pests, reducing the need for chemical pesticides.

Role in Nutrient Cycling

• Marine Ecosystems
  • Viruses play a crucial role in marine nutrient cycles by lysing microbial cells and releasing organic matter.
  • Example: Marine viruses infect and lyse phytoplankton and bacteria, releasing nutrients back into the water and supporting the marine food web.

Medical and Biotechnological Applications

• Gene Therapy
  • Viruses are engineered to deliver therapeutic genes to treat genetic disorders.
  • Example: Adeno-associated viruses (AAV) are used in gene therapy to treat conditions like spinal muscular atrophy.
• Vaccine Development
  • Viruses or their components are used to develop vaccines that protect against viral infections.
  • Example: The inactivated poliovirus is used in the polio vaccine.

Research Tools

• Molecular Biology Research
  • Viruses serve as tools to study gene function and regulation.
  • Example: Retroviruses are used to introduce genes into cells, allowing researchers to study gene expression and function.

How Do Viruses Get Into Your Body?

Viruses enter the body through various routes, including:

• Respiratory Tract: Inhalation of airborne droplets from coughs or sneezes (e.g., influenza, COVID-19).
• Gastrointestinal Tract: Ingestion of contaminated food or water (e.g., norovirus, rotavirus).
• Skin: Through cuts, bites, or direct contact with infected surfaces (e.g., rabies, Zika virus).
• Mucous Membranes: Contact with mucous membranes in the eyes, mouth, or genital areas (e.g., herpes simplex virus, HIV).

How Are Viruses Spread?

Viruses spread through several mechanisms:

• Person-to-Person Contact: Direct physical contact, such as touching or kissing.
• Airborne Transmission: Inhalation of respiratory droplets from an infected person’s cough or sneeze.
• Contaminated Surfaces: Touching surfaces contaminated with virus particles and then touching the face.
• Body Fluids: Exchange of bodily fluids, including blood, saliva, and semen (e.g., HIV, hepatitis B).
• Vectors: Transmission via insects like mosquitoes or ticks (e.g., dengue fever, West Nile virus).

How Do Viruses Cause Disease?

Viruses cause disease by:

• Infecting Host Cells: They invade host cells and take over the cellular machinery to replicate, often damaging or killing the host cells in the process.
• Immune Response: The body’s immune response to the viral infection can cause inflammation and other symptoms, contributing to the disease.
• Disrupting Normal Function: Some viruses produce toxins or cause cellular dysfunction, leading to symptoms and disease.

Role in Human Disease

Viruses play a significant role in human diseases, causing a wide range of illnesses, from mild infections to severe diseases:

• Common Colds and Influenza: Caused by rhinoviruses, coronaviruses, and influenza viruses.
• Chronic Infections: HIV causes AIDS, and hepatitis B and C can lead to chronic liver disease and cancer.
• Oncogenic Viruses: Some viruses, like human papillomavirus (HPV) and Epstein-Barr virus (EBV), can lead to cancer.
• Emerging Infectious Diseases: New viruses, such as SARS-CoV-2 (causing COVID-19), pose ongoing public health challenges.

Virus Reproduction

Virus reproduction, or replication, involves several key stages that allow viruses to invade host cells, hijack their machinery, and produce new viral particles. The general steps of the viral replication cycle include attachment, penetration, uncoating, replication, assembly, and release. Each virus follows a specific replication strategy, which can vary depending on whether the virus is a DNA virus, RNA virus, or retrovirus.

1. Attachment (Adsorption)

• Recognition and Binding: The virus attaches to specific receptors on the surface of the host cell. This interaction is highly specific, with viral proteins (often glycoproteins) recognizing and binding to complementary receptors on the host cell membrane.

2. Penetration (Entry)

• Methods of Entry:
  • Direct Fusion: Enveloped viruses, such as HIV, can fuse directly with the host cell membrane, releasing their nucleocapsid into the cytoplasm.
  • Endocytosis: Non-enveloped viruses and some enveloped viruses, like influenza, are engulfed by the host cell through a process called endocytosis, forming a vesicle inside the host cell.

3. Uncoating

• Release of Viral Genome: The viral capsid is removed, releasing the viral genetic material into the host cell’s cytoplasm. This step can occur simultaneously with or shortly after penetration.

4. Replication and Transcription

• Synthesis of Viral Components:
  • DNA Viruses: Typically replicate in the host cell’s nucleus using the host’s DNA polymerase (e.g., herpesviruses).
  • RNA Viruses: Usually replicate in the cytoplasm. Positive-sense RNA viruses can be directly translated by host ribosomes, while negative-sense RNA viruses must first be transcribed into positive-sense RNA by an RNA-dependent RNA polymerase (e.g., influenza viruses).
  • Retroviruses: Use reverse transcriptase to convert their RNA genome into DNA, which integrates into the host genome and is transcribed and replicated using the host’s machinery (e.g., HIV).

5. Assembly

• Formation of New Virions: New viral genomes and proteins are assembled into new viral particles. Capsid proteins form around the newly replicated viral nucleic acids, and in enveloped viruses, the new virions acquire their lipid envelope from the host cell membrane.

6. Release

• Exit from Host Cell:
  • Lysis: Non-enveloped viruses often cause the host cell to burst (lyse), releasing new virions.
  • Budding: Enveloped viruses typically exit the host cell by budding through the cell membrane, acquiring their envelope in the process without necessarily killing the host cell.

Characteristics of Viruses

Viruses are unique entities that blur the line between living and non-living things. They possess distinct characteristics that differentiate them from other microorganisms like bacteria and fungi.

1. Structure and Composition

• Size: Viruses are extremely small, ranging from 20 to 300 nanometers in diameter, which is much smaller than most bacteria.
• Shape: They come in various shapes, including helical, icosahedral, and complex structures.
• Components: Viruses consist of a nucleic acid core (either DNA or RNA) surrounded by a protein coat called a capsid. Some viruses also have an outer lipid envelope derived from the host cell membrane.

2. Genetic Material

• Types: Viral genomes can be either DNA or RNA, which can be single-stranded or double-stranded.
• Variability: The genetic material of viruses can vary greatly, leading to high mutation rates, especially in RNA viruses.

3. Host Dependency

• Obligate Intracellular Parasites: Viruses cannot replicate on their own. They must infect a host cell and hijack its machinery to produce new viral particles.
• Specificity: Viruses are often specific to their hosts, infecting particular species or even specific cell types within a species.

4. Reproduction

• Replication Cycle: The viral replication cycle includes attachment, penetration, uncoating, replication, assembly, and release.
  • Attachment: Virus attaches to the host cell surface.
  • Penetration: Virus or its genetic material enters the host cell.
  • Uncoating: Viral capsid is removed, exposing the viral genome.
  • Replication: Host cell’s machinery replicates the viral genome and synthesizes viral proteins.
  • Assembly: New viral particles are assembled from the replicated genome and synthesized proteins.
  • Release: New virions are released from the host cell, often causing cell death.

5. Lack of Cellular Structure

• Acellular: Viruses lack cellular components such as cytoplasm, organelles, and cell membranes found in living cells.
• Metabolism: They do not carry out metabolic processes like energy production or protein synthesis independently.

6. Pathogenicity

• Diseases: Viruses are responsible for a wide range of diseases in humans, animals, and plants. Common viral diseases include influenza, HIV/AIDS, COVID-19, and measles.
• Immune Response: The host immune system often mounts a response to viral infections, which can include the production of antibodies and activation of immune cells.

7. Mutability

• High Mutation Rates: Especially in RNA viruses, high mutation rates lead to rapid evolution and adaptability, contributing to challenges in developing long-lasting vaccines and treatments.
• Genetic Reassortment and Recombination: These processes can create new viral strains with different properties, such as increased virulence or drug resistance.

8. Transmission

• Modes: Viruses can be transmitted through various means, including direct contact, respiratory droplets, bodily fluids, vectors (like mosquitoes), and contaminated surfaces.
• Zoonotic Potential: Many viruses can cross species barriers, leading to zoonotic infections, where viruses jump from animals to humans.

Virus Shapes and Sizes

Virus Shapes

1. Icosahedral Viruses
   • Description: These viruses have a symmetrical, 20-sided structure with equilateral triangular faces.
   • Examples: Adenoviruses, Herpesviruses, Poliovirus.
2. Helical Viruses
   • Description: These viruses have a rod-like shape with a helical symmetry, where the viral RNA or DNA is coiled inside a cylindrical capsid.
   • Examples: Tobacco Mosaic Virus (TMV), Influenza viruses, Rabies virus.
3. Complex Viruses
   • Description: These viruses have a more intricate structure that doesn’t fit into the simple icosahedral or helical categories. Bacteriophages, for instance, have a head-tail structure.
   • Examples: T4 bacteriophage, Poxviruses (such as Variola virus causing smallpox).
4. Enveloped Viruses
   • Description: These viruses possess an outer lipid membrane derived from the host cell membrane, which surrounds their capsid. The shape can be spherical or pleomorphic (variable shape).
   • Examples: HIV, Influenza viruses, Herpesviruses.
5. Non-Enveloped Viruses
   • Description: These viruses lack an outer lipid membrane and have a capsid as their outer layer. They can be either icosahedral or helical.
   • Examples: Poliovirus, Adenovirus, Papillomavirus.

Virus Sizes

Viruses vary significantly in size, generally ranging from about 20 nanometers (nm) to 300 nanometers in diameter:

1. Small Viruses
   • Examples: Picornaviruses, such as Poliovirus, are about 20-30 nm in diameter.
   • Description: These viruses are among the smallest and often have simple icosahedral structures.
2. Medium-Sized Viruses
   • Examples: Influenza viruses are about 80-120 nm in diameter; Adenoviruses are about 70-90 nm.
   • Description: These viruses can have more complex structures, such as helical or icosahedral shapes, with or without envelopes.
3. Large Viruses
   • Examples: Herpesviruses, such as Herpes Simplex Virus, are about 150-200 nm in diameter; Poxviruses can be up to 300 nm.
   • Description: These viruses often have complex and enveloped structures, sometimes appearing larger due to their outer membrane.
4. Giant Viruses
   • Examples: Mimivirus and Pandoravirus can be over 400 nm in diameter and are visible under a light microscope.
   • Description: These viruses are exceptionally large, often containing more complex genetic material and structures than typical viruses.

Diseases Caused by Viruses

Human Diseases

1. Common Cold: Typically caused by rhinoviruses.
2. Influenza (Flu): Caused by influenza viruses.
3. COVID-19: Caused by the novel coronavirus SARS-CoV-2.
4. HIV/AIDS: Caused by the human immunodeficiency virus (HIV).
5. Hepatitis: Various types (A, B, C, D, E) caused by different hepatitis viruses.
6. Herpes: Caused by herpes simplex virus (HSV-1 and HSV-2).
7. Measles: Caused by the measles virus.
8. Mumps: Caused by the mumps virus.
9. Rubella (German Measles): Caused by the rubella virus.
10. Chickenpox and Shingles: Caused by the varicella-zoster virus.
11. Ebola: Caused by the Ebola virus.
12. Dengue Fever: Caused by the dengue virus.
13. Zika Virus: Caused by the Zika virus.
14. Rabies: Caused by the rabies virus.
15. Polio: Caused by the poliovirus.

Animal Diseases

1. Foot-and-Mouth Disease: Caused by the foot-and-mouth disease virus.
2. Avian Influenza (Bird Flu): Caused by avian influenza viruses.
3. Canine Parvovirus: Caused by the canine parvovirus.

Plant Diseases

1. Tobacco Mosaic Disease: Caused by the tobacco mosaic virus.
2. Tomato Yellow Leaf Curl Virus: Affects tomatoes and other plants.
3. Rice Tungro Disease: Caused by a combination of rice tungro bacilliform virus and rice tungro spherical virus.