Virology

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Virology


Generalized viral replication cycle
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Virology is the branch of biology, microbiology, medicine, and infectious disease that studies viruses, viral infections, viral replication, viral genetics, viral pathogenesis, antiviral drugs, vaccines, and the interaction between viruses and their host organisms. It is a major field within medical microbiology and has important applications in public health, epidemiology, immunology, molecular biology, genomics, oncology, veterinary medicine, plant pathology, and biotechnology.

File:Adenovirus transmission electron micrograph B82-0142 lores.jpg
Transmission electron micrograph of an adenovirus, a non-enveloped DNA virus.

Viruses are small infectious agents that require living cells to replicate. Unlike bacteria, fungi, and protozoa, viruses are not cellular organisms. A virus generally consists of a viral genome made of either DNA or RNA, surrounded by a protective protein coat called a capsid. Some viruses also possess a lipid viral envelope derived from host cell membranes. Virology examines how these agents enter cells, replicate, spread, evade the immune system, cause disease, and evolve.

Overview[edit]

Virology includes the study of both medically important viruses and viruses that infect animals, plants, fungi, bacteria, and archaea. Viruses that infect bacteria are known as bacteriophages or phages. Human and animal viruses are especially important in clinical medicine because they cause diseases ranging from mild common cold infections to life-threatening conditions such as rabies, viral hemorrhagic fever, AIDS, hepatitis, encephalitis, and viral pneumonia.

Major areas of virology include:

Definition of a virus[edit]

A virus is an infectious agent that can replicate only inside the living cells of a host. Viruses infect all forms of life, including animals, plants, bacteria, archaea, fungi, and protists. They are considered obligate intracellular parasites because they depend on host cell machinery for protein synthesis, energy metabolism, and many steps of replication.

A typical virus contains:

History of virology[edit]

The history of virology began with observations that some infectious diseases were caused by agents smaller than bacteria. In the late 19th century, scientists studying tobacco mosaic disease found that the infectious agent could pass through filters that trapped bacteria. This led to the concept of a "filterable virus."

Important milestones in virology include:

Virus structure[edit]

The structure of viruses varies greatly, but most viruses contain a genome enclosed within a protein shell. Viral structure is closely related to viral stability, transmission, host range, immune recognition, and disease severity.

Viral genome[edit]

The viral genome contains the genetic information needed to produce viral proteins and replicate the virus. Viral genomes may be:

Viral genomes may be linear, circular, segmented, or non-segmented. Segmented genomes, such as those of influenza virus, can undergo genetic reassortment, which may contribute to pandemic emergence.

Capsid[edit]

The capsid is the protein shell that protects the viral genome. It is made of repeating protein units called capsomeres. Capsids may have several types of symmetry:

The capsid helps protect the genome from environmental damage and may assist in attachment, entry, and delivery of the genome into host cells.

Viral envelope[edit]

An enveloped virus has a lipid membrane surrounding the capsid. The envelope is usually derived from host cell membranes during viral budding. It contains viral glycoproteins that mediate attachment and entry.

Examples of enveloped viruses include:

Enveloped viruses are often sensitive to drying, heat, detergents, alcohol, and disinfectants. Non-enveloped viruses are generally more resistant in the environment.

Viral surface proteins[edit]

Viral surface proteins are critical for host cell receptor binding, tissue tropism, immune recognition, and vaccine design. Examples include:

Classification of viruses[edit]

Viruses are classified based on genome type, replication strategy, structure, host range, and evolutionary relationships.

Baltimore classification[edit]

The Baltimore classification groups viruses according to the nature of their genome and how they produce messenger RNA. It is one of the most important systems in virology.

Group Genome type Examples
Group I Double-stranded DNA virus Herpesvirus, Adenovirus, Papillomavirus
Group II Single-stranded DNA virus Parvovirus
Group III Double-stranded RNA virus Rotavirus
Group IV Positive-sense single-stranded RNA virus Coronavirus, Flavivirus, Picornavirus
Group V Negative-sense single-stranded RNA virus Influenza virus, Rabies virus, Ebola virus, Measles virus
Group VI Single-stranded RNA retrovirus Human immunodeficiency virus
Group VII Double-stranded DNA reverse-transcribing virus Hepatitis B virus

Taxonomic classification[edit]

Formal virus taxonomy is organized by the International Committee on Taxonomy of Viruses. Viral classification may include:

Common virus families important in medicine include:

Viral replication[edit]

Viral replication is the process by which viruses produce new viral particles inside host cells. Although details vary among virus families, most viral replication cycles include several common steps.

Attachment[edit]

Attachment occurs when a virus binds to specific cell surface receptors on a susceptible host cell. Receptor binding helps determine tissue tropism, host range, and disease pattern.

Examples include:

Entry[edit]

Viruses enter cells by several mechanisms, including:

Enveloped viruses often enter by fusion of the viral envelope with a host membrane. Non-enveloped viruses may enter through endocytosis followed by capsid disassembly.

Uncoating[edit]

During uncoating, the viral genome is released from the capsid into the cytoplasm or nucleus. This step makes the genome available for transcription, translation, or replication.

Transcription and translation[edit]

Viruses must produce viral messenger RNA so that host ribosomes can synthesize viral proteins. DNA viruses often use host or viral DNA-dependent RNA polymerases. RNA viruses may need viral polymerases such as RNA-dependent RNA polymerase. Retroviruses use reverse transcriptase to convert RNA into DNA.

Genome replication[edit]

Viral genome replication depends on the virus type. DNA viruses often replicate in the cell nucleus, while many RNA viruses replicate in the cytoplasm. Exceptions exist, including poxviruses, which are DNA viruses that replicate in the cytoplasm.

Assembly[edit]

New viral genomes and proteins assemble into immature or mature virions. Assembly may occur in the nucleus, cytoplasm, or at cellular membranes.

Release[edit]

Viruses leave infected cells by:

Non-enveloped viruses often cause cell lysis. Enveloped viruses often bud from host membranes.

Viral genetics and evolution[edit]

Viruses evolve rapidly through mutation, recombination, reassortment, selection, and genetic drift. RNA viruses often have high mutation rates because many viral RNA polymerases lack proofreading activity.

Important mechanisms include:

Viral evolution is important in influenza pandemics, HIV drug resistance, SARS-CoV-2 variants, hepatitis C virus diversity, and emergence of zoonotic diseases.

Host range and tissue tropism[edit]

Host range refers to the species a virus can infect. Tissue tropism refers to the cell types or tissues a virus preferentially infects. These properties are influenced by:

  • Receptor availability
  • Host cell enzymes
  • Innate immune defenses
  • Body temperature
  • Tissue environment
  • Viral entry proteins
  • Host restriction factors

Examples include:

Viral pathogenesis[edit]

Viral pathogenesis is the process by which viruses cause disease. Disease may result from direct viral damage, immune-mediated injury, inflammation, persistent infection, oncogenic transformation, or systemic complications.

Mechanisms of viral disease include:

Clinical outcomes depend on viral virulence, infectious dose, route of exposure, host immunity, age, pregnancy, comorbidities, vaccination status, and genetic susceptibility.

Transmission of viruses[edit]

Viruses are transmitted through multiple routes. Understanding transmission is essential for infection control, public health, and epidemic prevention.

Respiratory transmission[edit]

Respiratory viruses spread through droplets, aerosols, and contaminated surfaces. Examples include:

Fecal-oral transmission[edit]

Fecal-oral viruses spread through contaminated food, water, hands, or surfaces. Examples include:

Bloodborne transmission[edit]

Bloodborne viruses spread through blood exposure, contaminated needles, transfusion, organ transplantation, or mucosal exposure. Examples include:

Sexual transmission[edit]

Sexually transmitted viruses include:

Vector-borne transmission[edit]

Vector-borne viruses are transmitted by arthropods such as mosquitoes, ticks, and sandflies. These viruses are often called arboviruses.

Examples include:

Zoonotic transmission[edit]

Zoonotic disease occurs when viruses are transmitted from animals to humans. Examples include:

Vertical transmission[edit]

Vertical transmission occurs from mother to child during pregnancy, childbirth, or breastfeeding. Examples include:

Immune response to viruses[edit]

The immune response to viral infections includes innate immunity, adaptive immunity, and immunological memory.

Innate immunity[edit]

The innate immune system provides rapid defense against viruses. Important components include:

Interferons induce an antiviral state in infected and neighboring cells by activating genes that inhibit viral replication.

Adaptive immunity[edit]

Adaptive immunity includes:

Protective immunity may prevent reinfection, reduce disease severity, or shorten illness duration.

Immune evasion[edit]

Viruses have evolved many strategies to evade immunity, including:

  • Antigenic variation
  • Latency
  • Downregulation of MHC class I
  • Inhibition of interferon signaling
  • Infection of immune cells
  • Production of immune-modulating proteins
  • Rapid mutation
  • Glycan shielding of viral surface proteins

Clinical virology[edit]

Clinical virology is the application of virology to the diagnosis, treatment, and prevention of viral diseases in patients. It overlaps with infectious disease, clinical microbiology, pathology, laboratory medicine, epidemiology, and public health.

Clinical virology includes:

  • Diagnosis of acute viral infection
  • Monitoring of chronic viral infection
  • Detection of antiviral resistance
  • Evaluation of vaccine-preventable diseases
  • Management of viral outbreaks
  • Infection prevention in hospitals
  • Screening blood and organ donors
  • Monitoring immunocompromised patients

Laboratory diagnosis of viral infections[edit]

File:Polymerase chain reaction.svg
Polymerase chain reaction is widely used in modern viral diagnosis.

Diagnosis of viral infections may involve direct detection of virus, detection of viral genetic material, detection of viral proteins, or detection of host immune responses.

Polymerase chain reaction[edit]

Polymerase chain reaction and related nucleic acid amplification tests are widely used to detect viral DNA or RNA. Reverse transcription polymerase chain reaction is used for RNA viruses.

PCR-based tests are important for:

Viral culture[edit]

Viral culture involves growing viruses in susceptible cells. It was historically central to virology but has been partly replaced by molecular tests. Culture remains useful for some viruses, research, and antiviral susceptibility testing.

Serology[edit]

Serology detects antibodies such as IgM and IgG produced in response to infection or vaccination. Serology is useful for diagnosing past exposure, immune status, or certain acute infections.

Examples include testing for:

Antigen testing[edit]

Antigen tests detect viral proteins. They are commonly used for rapid diagnosis of respiratory and gastrointestinal infections.

Examples include:

Electron microscopy[edit]

Electron microscopy can visualize viral particles but is less commonly used for routine diagnosis. It remains useful in research, outbreak investigation, and characterization of unknown agents.

Genome sequencing[edit]

Genome sequencing identifies viral genetic sequences and is useful for:

  • Tracking outbreaks
  • Identifying variants
  • Studying transmission chains
  • Detecting antiviral resistance
  • Investigating emerging viruses
  • Monitoring viral evolution

Major human viral diseases[edit]

Respiratory viral infections[edit]

Respiratory viruses are among the most common causes of human illness. Important conditions include:

Gastrointestinal viral infections[edit]

Viruses are major causes of gastroenteritis, especially in children and outbreaks.

Important viruses include:

Hepatitis viruses[edit]

Viral hepatitis is inflammation of the liver caused by viruses. Major hepatitis viruses include:

Hepatitis B and C can cause chronic infection, cirrhosis, liver failure, and hepatocellular carcinoma.

Neurotropic viruses[edit]

Neurotropic viruses infect the nervous system and may cause meningitis, encephalitis, myelitis, or peripheral nerve disease.

Examples include:

Skin and mucosal viral infections[edit]

Viruses commonly affect the skin and mucous membranes. Examples include:

Sexually transmitted viral infections[edit]

Important sexually transmitted viral infections include:

Congenital viral infections[edit]

Congenital viral infections can affect fetal development and newborn health. Important examples include:

Viral hemorrhagic fevers[edit]

Viral hemorrhagic fevers are severe illnesses associated with fever, vascular dysfunction, bleeding, shock, and multiorgan involvement.

Examples include:

Oncogenic viruses[edit]

Some viruses contribute to the development of cancer. These are known as oncoviruses or oncogenic viruses.

Important oncogenic viruses include:

Viral oncogenesis may involve chronic inflammation, integration into the host genome, expression of viral oncogenes, immune suppression, or disruption of tumor suppressor pathways.

Persistent, latent, and chronic viral infections[edit]

Not all viral infections are acute and self-limited. Some viruses persist in the body for months, years, or lifelong.

Latent infection[edit]

In latency, the viral genome remains in host cells with limited gene expression and can reactivate later.

Examples include:

Chronic infection[edit]

Chronic viral infections involve ongoing viral replication or persistence.

Examples include:

Slow viral infection[edit]

Some infections have long incubation periods and progressive disease. Examples include certain prion diseases, although prions are not viruses, and rare viral conditions such as progressive multifocal leukoencephalopathy caused by JC virus.

Antiviral therapy[edit]

File:Zidovudine.svg
Zidovudine is an antiretroviral drug used in the treatment of HIV infection.

Antiviral drugs inhibit viral replication or viral spread. Antiviral therapy differs from antibiotic therapy because viruses use host cellular machinery, making selective drug targeting more difficult.

Major classes of antiviral drugs include:

Examples include:

Antiviral resistance[edit]

Antiviral resistance occurs when viruses acquire mutations that reduce susceptibility to antiviral drugs. Resistance is especially important in chronic infections such as HIV, hepatitis B, hepatitis C, cytomegalovirus, and influenza.

Mechanisms of resistance include:

  • Mutation in viral polymerase
  • Mutation in viral protease
  • Mutation in reverse transcriptase
  • Altered drug binding site
  • Increased viral fitness under drug pressure
  • Poor adherence to therapy
  • Inadequate drug levels

Resistance testing may guide therapy in HIV and selected other viral infections.

Vaccines and prevention[edit]

Vaccination is one of the most important achievements of virology and public health. Vaccines stimulate protective immunity without causing the disease they are designed to prevent.

Types of viral vaccines include:

Important viral vaccines include vaccines against:

Infection control[edit]

Prevention of viral transmission depends on the route of spread. Measures include:

In healthcare settings, viral infection control is especially important for influenza, COVID-19, norovirus, respiratory syncytial virus, measles, varicella, hepatitis B, hepatitis C, and HIV.

Emerging and re-emerging viruses[edit]

Emerging infectious diseases include newly recognized viruses, viruses spreading to new regions, and viruses increasing in incidence. Re-emerging viruses are known viruses that return after a period of decline.

Factors contributing to viral emergence include:

Examples include:

Virology and public health[edit]

Virology is essential to public health because viral infections can cause outbreaks, epidemics, and pandemics. Public health virology includes surveillance, diagnostic testing, vaccination campaigns, outbreak investigation, risk communication, and development of infection control policies.

Key public health activities include:

  • Monitoring viral disease trends
  • Detecting outbreaks
  • Sequencing viral genomes
  • Tracking variants
  • Evaluating vaccine effectiveness
  • Preventing healthcare-associated infections
  • Screening blood and organ donations
  • Preparing for pandemics
  • Educating the public about prevention

Virology research methods[edit]

Virologists use a wide range of laboratory and computational methods.

Cell culture[edit]

Cell culture is used to grow viruses, study replication, test antiviral drugs, and produce vaccines.

Animal models[edit]

Animal models help researchers study viral pathogenesis, immune responses, transmission, and vaccine safety. Common models include mice, ferrets, hamsters, nonhuman primates, and other species.

Molecular cloning[edit]

Molecular cloning allows manipulation of viral genomes and viral genes for research and vaccine development.

Reverse genetics[edit]

Reverse genetics systems allow scientists to generate viruses from cloned genetic material. This is useful for studying viral gene function, attenuation, vaccine development, and pathogenesis.

Metagenomics[edit]

Metagenomics identifies viral sequences directly from environmental, animal, or clinical samples without needing to culture the virus.

Bioinformatics[edit]

Bioinformatics is used to analyze viral genomes, identify mutations, study evolution, track outbreaks, and predict protein structure.

Virology in biotechnology and medicine[edit]

Viruses are not only pathogens; they are also useful tools in medicine and biotechnology.

Applications include:

  • Viral vectors for gene therapy
  • Oncolytic virus therapy for cancer
  • Phage therapy for bacterial infections
  • Vaccine development
  • Molecular biology tools
  • Protein expression systems
  • Viral delivery of genetic material
  • CRISPR-associated delivery platforms
  • Cancer immunotherapy research

Bacteriophages[edit]

Bacteriophages are viruses that infect bacteria. They are among the most abundant biological entities on Earth and are important in microbial ecology, bacterial evolution, and biotechnology.

Bacteriophages may have:

Applications of bacteriophages include:

  • Phage therapy
  • Food safety
  • Bacterial typing
  • Genetic engineering
  • Study of microbial communities
  • Control of bacterial contamination

Special populations[edit]

Children[edit]

Children are commonly affected by viral infections such as respiratory syncytial virus, rotavirus, influenza, enterovirus, varicella, measles, and hand, foot, and mouth disease. Vaccination is central to pediatric viral disease prevention.

Pregnant patients[edit]

Viral infections during pregnancy can affect the pregnant patient, fetus, or newborn. Important viruses include rubella virus, cytomegalovirus, Zika virus, hepatitis B virus, HIV, varicella-zoster virus, influenza virus, and herpes simplex virus.

Immunocompromised patients[edit]

Patients with impaired immunity, including transplant recipients, cancer patients, people with HIV, and those receiving immunosuppressive therapy, are at increased risk of severe or persistent viral infections. Important viruses include cytomegalovirus, Epstein-Barr virus, BK virus, JC virus, varicella-zoster virus, respiratory viruses, and adenovirus.

Differential diagnosis[edit]

Viral infections may resemble bacterial, fungal, parasitic, inflammatory, autoimmune, or toxic conditions. Differential diagnosis depends on the clinical syndrome.

Examples include:

Relationship to other fields[edit]

Virology overlaps with many scientific and medical disciplines:

Common terms in virology[edit]

Term Meaning
Virion Complete infectious viral particle
Capsid Protein shell surrounding the viral genome
Envelope Lipid membrane surrounding some viruses
Genome Genetic material of the virus
Tropism Preference of a virus for particular hosts, tissues, or cells
Latency Dormant persistence of a virus with potential reactivation
Viremia Presence of virus in the bloodstream
Cytopathic effect Cell damage caused by viral infection
Antigenic drift Gradual antigenic change caused by mutation
Antigenic shift Major antigenic change, often through reassortment
Zoonosis Infection transmitted from animals to humans
Pandemic Epidemic occurring across countries or continents

Examples of medically important viruses[edit]

Virus Family Major disease association
Human immunodeficiency virus Retroviridae HIV infection, AIDS
Influenza virus Orthomyxoviridae Influenza, viral pneumonia
SARS-CoV-2 Coronaviridae COVID-19
Hepatitis B virus Hepadnaviridae Hepatitis B, cirrhosis, hepatocellular carcinoma
Hepatitis C virus Flaviviridae Hepatitis C, cirrhosis, hepatocellular carcinoma
Herpes simplex virus Herpesviridae Oral herpes, genital herpes, herpes encephalitis
Varicella-zoster virus Herpesviridae Chickenpox, shingles
Epstein-Barr virus Herpesviridae Infectious mononucleosis, Burkitt lymphoma
Cytomegalovirus Herpesviridae Congenital infection and disease in immunocompromised patients
Human papillomavirus Papillomaviridae Warts, cervical cancer, oropharyngeal cancer
Rabies virus Rhabdoviridae Rabies
Ebola virus Filoviridae Ebola virus disease
Dengue virus Flaviviridae Dengue fever
Zika virus Flaviviridae Zika fever, congenital Zika syndrome
Norovirus Caliciviridae Viral gastroenteritis
Rotavirus Reoviridae Severe gastroenteritis in children

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