Virology: Introduction, Diagram, Stages, Types, Life Cycle, FAQs

Introduction To Virology

Virology is the subfield of microbiology that deals with the study of viruses in structure, classification, genetics, and mechanisms of infection and replication in host organisms. The study of viruses is of paramount importance because they are the potential causes of a large number of diseases in human, animal, and plant organisms while playing such important roles in ecology and evolution.

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The discovery of viruses dates back to the end of the 19th century with the discovery of the tobacco mosaic virus. Since then, several important breakthroughs, among them electron microscopy and molecular biology techniques, have propelled the field ahead. Viral diseases, from influenza to HIV/AIDS to COVID-19, underline the part of virology plays in public health and spur further research into mechanisms aimed at vaccines, antiviral drugs, and outbreak prevention and control strategies.

Structure And Classification Of Viruses

Viruses are made of a few structures which play a crucial role in infecting host cells and self-replication. The simple structure includes the capsid, the genetic material, and sometimes the outer envelope.

The capsid is the protein shell that covers and protects the genetic material of the virus. It is made of protein subunits known as capsomeres, and they have helical, icosahedral, or complex shapes.

Their genetic material can either be DNA or RNA. It may also be single-stranded or double-stranded. Contained within this genetic material is the information on the replication of the virus and the manufacture of new viral particles inside the host cell.

Some viruses possess an envelope, composed of a membrane of lipids from the host cellular membrane. This envelope surrounds the capsid and often contains viral glycoproteins that contribute to attachment and entry into host cells. The presence or absence of an envelope has a bearing on virus stability, infectivity, and transmission.

Classification Of Viruses

They are classified based on many characteristics, including their viral genetic material, the presence or absence of the envelope, and a lot of other structural and functional properties.

DNA Viruses vs. RNA Viruses

• DNA Viruses: These are viruses where their genetic material is represented by DNA. Examples are the ones of the Herpesviridae family, which causes herpes simplex and varicella-zoster and the Adenoviridae family, responsible for respiratory infections.

• RNA Viruses: These include viruses with RNA as their genetic material. This grouping includes viruses of the Retroviridae family, which includes HIV, and the Orthomyxoviridae family, the causative agent of influenza.

Enveloped vs. Non-enveloped Viruses

• Enveloped viruses have capsids enveloped by a lipid envelope originating from the host cell membrane and containing viral-derived glycoproteins. Examples of these include the families Herpesviridae and Retroviridae viruses.

• Non-enveloped Viruses have no lipid envelope and thus are generally more resistant to environmental conditions. Examples include the family Picornaviridae, which includes poliovirus and rhinovirus and is the causative agent of the common cold, and the family Adenoviridae.

Types Of Viruses

Viruses can also be classified based on the type of host they infect and transmission methods.

• Animal Viruses infect animals, including humans. An example is the influenza virus, the rabies virus, and the coronavirus responsible for COVID-19.
• Plant Viruses infect plants, leading to diseases that most generally affect crop yield and quality. Examples include the tobacco mosaic virus and the cucumber mosaic virus.
• Bacteriophages, or phages, infect bacteria. Examples would include T4 phages that infect bacteria such as E. coli.
• Retroviruses are RNA viruses that make use of reverse transcriptase, a single-stranded RNA that duplicates into DNA which then integrates into the genome of the host. One such famous retrovirus is the HIV virus, which causes AIDS.
• Arboviruses are transmitted by arthropod vectors, sometimes infecting both humans and animals. Diseases like dengue fever, Zika virus and West Nile virus are some of those caused by these viruses.

Viral Replication Cycle

Several phases are involved in the replication of the virus, starting with attachment, by which the virus binds to the host cell surface via specific receptors. Then there is penetration, wherein the virus or its genetic material enters the host cell. This is followed by uncoating, which releases the viral genome from the capsid. Then begins the process of replication and transcription, during which the viral genome is copied and, subsequently, the viral proteins are translated. Assembly means, together with the packaging of viral genomes into new capsids. In the last step, newly produced virions leave the host cell, normally through host cell lysis or budding off in the case of enveloped viruses.

DNA viruses replicate in the nucleus and make use predominantly of the host replication machinery. RNA viruses normally replicate in the cytoplasm and mostly carry their polymerases with them for transcription and replication.

Diagram Of The Viral Replication Cycle

The diagram given below shows the steps involved in the replication process of a virus.

Host-Virus Interactions

Various mechanisms, such as receptor-mediated endocytosis, direct fusion with the cell membrane, or by directly injecting their genetic material into the host cell, mediate the entry of viruses into cells. Following the entry into host cells, an infection provokes an immune response to the virus. Innate immunity presents the initial line of defence through physical barriers, phagocytic cells, and interferons. This is followed by adaptive immunity, mediated by T cells killing the infected cells and B cells synthesising and secreting antibodies against the virus.

Different mechanisms of defying immune responses have been evolved by viruses. For instance, HIV causes depletion of the immune system through infection and depletion of CD4+ T cells. Influenza virus embodies continuous antigenic shifts and drifts to slip from antibody recognition. These interactions underline, at a higher level of generality, how these are in the eternal race between immune defences and viral evasion strategies.

Viral Pathogenesis

Viral pathogenesis is described as the mechanisms by which viruses create a diseased state in their hosts. Virus-mediated cellular lysis processing occurs directly through the killing of infected cells upon the release of new viral particles. The immune-mediated damage of tissues thereby occurs inadvertently due to actions by the immune response against the infection. Oncogenesis is a process related to uncontrolled cell division, leading to cancer, as experienced with oncogenic viruses such as human papillomavirus. They include the hepatitis viruses, which cause inflammation of the liver and eventually damage it. This leads to potential cirrhosis or liver cancer, and the oncogenic viruses, such as EBV and HPV, which are associated with cancers like Burkitt's lymphoma and that of the cervix respectively.

Diagnosis And Treatment Of Viral Infections

Diagnosis of viral infection includes PCR, a technique that identifies viral genetic material with a high degree of sensitivity. Serology, which detects antibodies or antigens in the blood. Viral culture is a process of growing the virus under controlled laboratory conditions. Antiviral drugs target the inhibition of the replication of the virus and include Acyclovir, applied against herpes infections, and antiretrovirals against HIV.

Viral diseases are prevented by vaccines. The vaccines developed can be considered under three headings: live attenuated ones, like MMR for measles, mumps, and rubella, consisting of the live attenuated viruses. Inactivated ones that kill viruses, such as the polio vaccine. There is also the subunit type of vaccine, which constitutes parts of the virus itself, such as the HPV vaccine. These strategies have significantly lowered the incidence and seriousness of many viral infections.

Emerging And Re-Emerging Viruses

A route for the rise of a new pathogen is zoonotic transmission from animals to humans because of the increased frequency of contact between humans and animals. Changing environmental conditions, such as deforestation and climate change, can lead to habitat changes and, consequently, may act through vector-borne diseases. Examples of Ebola and Zika underline the speed at which emergent viruses can spread themselves at a global level and cause effects, posing a challenge for the potential response to public health. Strategies put in place to counter the threats of these pathogens include active surveillance systems, rapid response protocols in case of a breakout event, and research into preventive vaccines and curative treatments.

The video describing Virology is given below