{"id":4017,"date":"2026-01-04T12:16:07","date_gmt":"2026-01-04T06:46:07","guid":{"rendered":"https:\/\/vajiramandravi.com\/upsc-exam\/?p=4017"},"modified":"2026-01-05T11:34:11","modified_gmt":"2026-01-05T06:04:11","slug":"virus","status":"publish","type":"post","link":"https:\/\/vajiramandravi.com\/upsc-exam\/virus\/","title":{"rendered":"Virus – Types, Structure, and Examples, UPSC Prelims 2025"},"content":{"rendered":"

Viruses are\u00a0parasitic entities lacking cells,<\/strong>\u00a0making them unclassifiable within any specific kingdom. They are sub-microscopic infectious agents that can only replicate inside host cells of living organisms. They range from\u00a020 to 400 nm<\/strong>\u00a0and can infect all life forms -\u00a0humans, animals, plants, bacteria.<\/strong>\u00a0They are found in almost every ecosystem and environment on Earth. Though\u00a0not classified as living organisms,<\/strong>\u00a0viruses\u00a0exhibit biological properties<\/strong>\u00a0when inside a host cell.<\/p>\r\n

Many fatal epidemics throughout history like\u00a0smallpox, influenza, HIV\/AIDS, Coronavirus,\u00a0<\/strong>etc. have been caused by viruses. Hence, understanding the nature, structure, and replication of viruses is essential for developing diagnostic tests, vaccines, and antiviral drugs.<\/p>\r\n

About Viruses<\/h2>\r\n

Viruses are\u00a0non-cellular<\/strong>\u00a0entities with genetic material enclosed in a protein coat called\u00a0capsid<\/strong>. The viral genome is usually either RNA or DNA. Outside the host, viruses remain\u00a0inert<\/strong>. But once\u00a0inside host cells<\/strong>, they utilise the cell machinery to make\u00a0copies of themselves<\/strong>, often destroying the host cell. Viruses has following properties:<\/p>\r\n

    \r\n\t
  • Viruses, including human pathogens like enteroviruses and caliciviruses, survive in marine environments.\u00a0<\/li>\r\n\t
  • Viruses can infect bacteria. Bacteriophages (phages) are viruses that specifically infect bacteria. They inject their genetic material into bacterial cells, hijack their machinery, and often lyse the host.<\/span><\/li>\r\n\t
  • Viruses like HIV, HBV, and EBV alter host transcriptional activity by modifying DNA methylation, recruiting chromatin-modifying enzymes, or disrupting transcription factors. These changes promote viral replication or disease progression.<\/span><\/li>\r\n<\/ul>\r\n
    \"Virus<\/figure>\r\n

    Discovery of Virus<\/strong><\/h3>\r\n

    The concept of a disease-causing entity smaller than bacteria was first hypothesized in the 1890s.<\/p>\r\n

      \r\n\t
    • Discovery of Mosaic Disease (1892)<\/strong>: Dmitri Ivanovsky demonstrates that sap from tobacco plants with mosaic disease remains infectious after bacterial filtration.<\/li>\r\n\t
    • Term \"Virus<\/strong>\" (1898): Martinus Beijerinck<\/strong>\u00a0introduces the term\u00a0\"virus\"<\/strong>\u00a0to describe the newly identified soluble and filterable infectious agent.<\/li>\r\n\t
    • Advancements in Virus Study:\u00a0<\/strong>Viruses grown in plants and animals for pathogenesis research.<\/li>\r\n\t
    • Confirmation of Viral Particulate Nature:\u00a0<\/strong>Wendell Stanley crystallises the tobacco mosaic virus, confirming its particulate nature and persistent infectivity.<\/li>\r\n\t
    • Technological Advancements:<\/strong>\u00a0The invention of the\u00a0electron microscope<\/strong>\u00a0enabled the visualisation of viruses. The biochemical analysis allowed the determination of viral chemical composition.\r\n\r\n
        \r\n\t
      • Improvements in\u00a0virus isolation\u00a0<\/strong>and\u00a0detection<\/strong>\u00a0methods resulted in the discovery of several important human viruses.<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ul>\r\n
        \"Virus<\/figure>\r\n

        Structure of Viruses<\/h2>\r\n

        Viruses are\u00a0unique parasites<\/strong>, not classified as plants, animals, or bacteria. Despite their reproductive abilities, viruses aren't living organisms; they rely on host cells for survival. They lack ribosomes and energy-producing mechanisms, relying on host cells for protein synthesis and energy. Viruses exhibit tremendous diversity in size, shape, and composition, but some common structural elements are:<\/p>\r\n

          \r\n\t
        • Genetic Material:\u00a0<\/strong>This can be single-stranded DNA, double-stranded DNA, single-stranded RNA, or double-stranded RNA. The size of the genome ranges from a few thousand bases for small viruses to over a million bases for giant viruses.<\/li>\r\n\t
        • Capsid:\u00a0<\/strong>This is a\u00a0protective layer of protein\u00a0<\/strong>surrounding the genome. It consists of multiple copies of capsomere subunits tightly arranged to form a shell.\r\n\r\n
            \r\n\t
          • Based on symmetry,\u00a0capsids\u00a0<\/strong>can be\u00a0helical, icosahedral,\u00a0<\/strong>or\u00a0complex.<\/strong><\/li>\r\n<\/ul>\r\n<\/li>\r\n\t
          • Envelope:\u00a0<\/strong>Some groups of viruses have a membrane envelope made of\u00a0lipids, proteins,\u00a0<\/strong>and\u00a0glycoproteins.<\/strong>\u00a0Envelopes are acquired by budding from host cell membranes. They mediate viral entry and immune evasion.<\/li>\r\n\t
          • Envelope Protein (Spikes):Glycoprotein<\/strong>\u00a0spikes protruding from the envelope help attach and enter host cells.\u00a0Influenza\u00a0<\/strong>and\u00a0HIV<\/strong>\u00a0viruses contain these spike structures.<\/li>\r\n\t
          • Morphology:\u00a0<\/strong>Based on their morphology, viruses can be classified as naked icosahedral, enveloped icosahedral, naked helical, and enveloped helical.<\/li>\r\n\t
          • Bacteriophages<\/strong>\u00a0infecting bacteria exhibit\u00a0head-tail\u00a0<\/strong>structures.<\/li>\r\n<\/ul>\r\n

            Difference between Virus and Bacteria<\/strong><\/h3>\r\n
            \r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
            Parameter<\/strong><\/td>\r\nVirus<\/strong><\/td>\r\nBacteria<\/strong><\/td>\r\n<\/tr>\r\n
            Type<\/strong><\/td>\r\nAcellular<\/td>\r\nUnicellular organism<\/td>\r\n<\/tr>\r\n
            Size<\/strong><\/td>\r\n20 - 400 nm<\/td>\r\n0.2 - 10 \u03bcm<\/td>\r\n<\/tr>\r\n
            Structure<\/strong><\/td>\r\nCapsid with genetic material<\/td>\r\nCell wall, cell membrane, cytoplasm, DNA<\/td>\r\n<\/tr>\r\n
            Replication<\/strong><\/td>\r\nRequire host cells<\/td>\r\nCan replicate independently<\/td>\r\n<\/tr>\r\n
            Treatment<\/strong><\/td>\r\nAntiviral drugs<\/td>\r\nAntibiotics<\/td>\r\n<\/tr>\r\n
            Diseases caused<\/strong><\/td>\r\nCOVID-19, influenza, HIV, smallpox<\/td>\r\nTuberculosis, pneumonia, tetanus<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/figure>\r\n

            Classification of Viruses<\/h2>\r\n

            They contain only a few elements by which they can be classified: the viral genome, the type of capsid, and the envelope structure for the enveloped viruses.<\/p>\r\n

              \r\n\t
            • Host Organism Infected:<\/strong>\r\n
                \r\n\t
              • Animal Viruses, Plant Viruses, Bacteriophages<\/li>\r\n\t
              • Based on structure:\u00a0<\/strong>Viruses can be classified into four types:<\/li>\r\n\t
              • Helical<\/strong>: Long spiral capsids e.g.\u00a0Influenza<\/strong><\/li>\r\n\t
              • Icosahedral<\/strong>: Polygonal capsids with 20 faces e.g.\u00a0Poliovirus<\/strong><\/li>\r\n\t
              • Enveloped<\/strong>: Lipid bilayer covering capsid e.g. HIV,\u00a0Coronavirus<\/strong><\/li>\r\n\t
              • Complex<\/strong>: Intricate structures e.g. Bacteriophages (viruses that infect bacteria.<\/li>\r\n<\/ul>\r\n<\/li>\r\n\t
              • The viral genome is tightly packed into a small volume within the capsid. Animal viruses have genome sizes ranging from\u00a03,000 to 1,000,000\u00a0<\/strong>nucleotide bases.<\/li>\r\n\t
              • Nature of Nucleic Acid:<\/strong>\r\n
                  \r\n\t
                • DNA Viruses<\/strong>: Adenovirus, Herpesvirus, Hepadnavirus<\/li>\r\n\t
                • RNA Viruses:<\/strong>\u00a0Picornavirus, Orthomyxovirus, Rhabdovirus<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ul>\r\n

                  Viruses are classified by the\u00a0Baltimore system<\/strong>\u00a0based on their type of genetic material. There are seven groups:<\/p>\r\n

                  \r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
                  Group<\/strong><\/td>\r\nNucleic Acid<\/strong><\/td>\r\nExample Viruses<\/strong><\/td>\r\n<\/tr>\r\n
                  I<\/strong><\/td>\r\ndsDNA*<\/strong><\/td>\r\nAdenovirus, Herpesvirus, Poxvirus<\/td>\r\n<\/tr>\r\n
                  II<\/strong><\/td>\r\nssDNA*<\/strong><\/td>\r\nParvovirus<\/td>\r\n<\/tr>\r\n
                  III<\/strong><\/td>\r\ndsRNA<\/strong><\/td>\r\nReovirus<\/td>\r\n<\/tr>\r\n
                  IV<\/strong><\/td>\r\n(+)ssRNA<\/strong><\/td>\r\nPicornavirus, Togavirus<\/td>\r\n<\/tr>\r\n
                  V<\/strong><\/td>\r\n(-)ssRNA<\/strong><\/td>\r\nOrthomyxovirus, Rhabdovirus<\/td>\r\n<\/tr>\r\n
                  VI<\/strong><\/td>\r\nssRNA-RT*<\/strong><\/td>\r\nRetrovirus - HIV<\/td>\r\n<\/tr>\r\n
                  VII<\/strong><\/td>\r\ndsDNA-RT<\/strong><\/td>\r\nHepadnavirus - Hepatitis B<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/figure>\r\n

                  (*ds - Double stranded, *ss - Single-stranded, *RT - Reverse transcribing)<\/p>\r\n

                  This genetic material of viruses encodes proteins and enzymes vital for viral replication inside host cells.<\/p>\r\n

                  Reproduction in Viruses<\/h2>\r\n

                  They cannot replicate on their own but rather depend on their host cell's protein synthesis pathways to reproduce. There are two processes used by viruses to replicate:<\/p>\r\n

                    \r\n\t
                  • Lytic Cycle:\u00a0<\/strong>This is the productive viral replication cycle exemplified by bacteriophages. It has the following stages:\r\n\r\n
                      \r\n\t
                    • Attachment<\/strong>: Viral spikes bind to specific receptors on the host cell surface<\/li>\r\n\t
                    • Penetration<\/strong>: The viral genome enters into the cell cytoplasm through endocytosis or membrane fusion.<\/li>\r\n\t
                    • Replication<\/strong>: Viral genes and genome hijack the cell machinery to produce viral proteins and nucleic acid.<\/li>\r\n\t
                    • Assembly<\/strong>: New viral particles are assembled using the replicated components.<\/li>\r\n\t
                    • Release<\/strong>: Cloned viruses break open the host cell by lysis and spread the infection.<\/li>\r\n\t
                    • Through this cycle, a virus can produce thousands of progenies from a single host cell in a matter of hours.<\/li>\r\n<\/ul>\r\n<\/li>\r\n\t
                    • Lysogenic Cycle:\u00a0<\/strong>In this cycle, the viral genome gets incorporated into the host DNA. It persists in a dormant state as a\u00a0prophage without replication<\/strong>.\r\n\r\n
                        \r\n\t
                      • \u00a0The\u00a0prophage<\/strong>\u00a0later enters the\u00a0lytic cycle<\/strong>\u00a0in response to environmental triggers.<\/li>\r\n\t
                      • In latent infections like\u00a0herpes,<\/strong>\u00a0the viral genome is maintained in host cells in inactive form for later reactivation.<\/li>\r\n\t
                      • Retroviruses\u00a0<\/strong>like HIV integrate their genome into the host DNA.<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ul>\r\n
                        \"Lytic<\/figure>\r\n

                        Applications and Significance of Viruses<\/h2>\r\n

                        Scientists have harnessed the biology of viruses for the development of new tools and applications in molecular biology, medicine, and other streams. Despite the havoc they create, viruses also have some beneficial applications:<\/p>\r\n

                          \r\n\t
                        • Vaccine development<\/strong>: Attenuated or inactive viruses are used to produce vaccines and immunize against viral diseases.\r\n\r\n
                            \r\n\t
                          • For example, the\u00a0Oral polio vaccine (OPV) from attenuated poliovirus<\/strong>\u00a0led to the near eradication of\u00a0poliomyelitis.<\/strong><\/li>\r\n<\/ul>\r\n<\/li>\r\n\t
                          • Gene therapy vectors<\/strong>: Viruses can transfer genetic material into cells, enabling gene therapy.\r\n\r\n
                              \r\n\t
                            • For example, the\u00a0Adeno<\/strong>-associated virus<\/strong>\u00a0(AAV) has been used in clinical trials for treating\u00a0blindness, haemophilia, and neurodegenerative diseases.<\/strong><\/li>\r\n<\/ul>\r\n<\/li>\r\n\t
                            • Cancer treatment<\/strong>: Viruses specifically able to infect and kill cancer cells are being studied as oncolytic viruses for cancer treatment.\r\n\r\n
                                \r\n\t
                              • An example is the\u00a0herpesvirus T-Vec<\/strong>\u00a0which is FDA-approved for treating\u00a0melanoma.<\/strong><\/li>\r\n<\/ul>\r\n<\/li>\r\n\t
                              • Bacteriophages<\/strong>: Viruses infecting bacteria called bacteriophages have been utilised for killing strains of\u00a0pathogenic bacteria\u00a0<\/strong>like treating\u00a0antibiotic-resistant superbugs.<\/strong>\r\n
                                  \r\n\t
                                • Phage therapy cocktails<\/strong>\u00a0are commercially produced and prescribed in Russia and parts of Europe.<\/li>\r\n<\/ul>\r\n<\/li>\r\n\t
                                • Biological control<\/strong>: Certain viruses attacking insect pest species can be formulated as biopesticides for controlling costly crop infestations.\r\n\r\n
                                    \r\n\t
                                  • An example is the use of baculoviruses against\u00a0gipsy moth infestations<\/strong>\u00a0in North America.<\/li>\r\n<\/ul>\r\n<\/li>\r\n\t
                                  • Molecular biology<\/strong>: Due to their\u00a0structural simplicity<\/strong>, viruses serve as excellent model systems and tools to examine fundamental cellular processes like\u00a0gene expression, genome replication, viral-host interactions,<\/strong>\u00a0etc. that provide wider biological insights.\r\n\r\n
                                      \r\n\t
                                    • Studies of\u00a0tobacco mosaic virus<\/strong>\u00a0led to the discovery of the\u00a0first virus genome enabling advances<\/strong>\u00a0in molecular biology.<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ul>\r\n
                                      \"Virus<\/figure>\r\n

                                      Diseases Caused by Viruses<\/h2>\r\n

                                      Viruses account for several troublesome diseases in humans ranging from mild ailments to severe life-threatening conditions.<\/p>\r\n

                                      \r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
                                      Virus Families<\/strong><\/td>\r\nDiseases<\/strong><\/td>\r\nMode of Transmission<\/strong><\/td>\r\n<\/tr>\r\n
                                      Orthomyxoviridae<\/strong><\/td>\r\nInfluenza, bird flu<\/td>\r\nAirborne route<\/td>\r\n<\/tr>\r\n
                                      Picornaviridae<\/strong><\/td>\r\nPoliomyelitis, common cold, hepatitis A<\/td>\r\nFaeco-oral route<\/td>\r\n<\/tr>\r\n
                                      Togaviridae<\/strong><\/td>\r\nRubella, chikungunya<\/td>\r\nMosquito vectors<\/td>\r\n<\/tr>\r\n
                                      Flaviviridae<\/strong><\/td>\r\nDengue, yellow fever, hepatitis C<\/td>\r\nMosquito vectors<\/td>\r\n<\/tr>\r\n
                                      Retroviridae<\/strong><\/td>\r\nHIV\/AIDS<\/td>\r\nBody fluids, blood<\/td>\r\n<\/tr>\r\n
                                      Herpesviridae<\/strong><\/td>\r\nHerpes, chickenpox, shingles<\/td>\r\nContact, sexual, saliva<\/td>\r\n<\/tr>\r\n
                                      Hepadnaviridae<\/strong><\/td>\r\nHepatitis B<\/td>\r\nBlood, body fluids<\/td>\r\n<\/tr>\r\n
                                      Paramyxoviridae<\/strong><\/td>\r\nMeasles, mumps<\/td>\r\nAirborne route<\/td>\r\n<\/tr>\r\n
                                      Rhabdoviridae<\/strong><\/td>\r\nRabies<\/td>\r\nThe bite from an infected host<\/td>\r\n<\/tr>\r\n
                                      Poxviridae<\/strong><\/td>\r\nSmallpox<\/td>\r\nClose Contact<\/td>\r\n<\/tr>\r\n
                                      Coronaviridae<\/strong><\/td>\r\nSARS, MERS, COVID-19<\/td>\r\nRespiratory droplets<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/figure>\r\n

                                      Viruses are unique microscopic infective agents that replicate only within host cells. Though tiny in size, they have an outsized impact on ecology, evolution, and human disease. Developing antiviral strategies is an ongoing battle due to the immense diversity and mutability of viruses as parasites dependent on hosts. Virology research on emerging viral threats, and developing new-generation vaccines and antiviral drugs will remain priorities for improving public health outcomes.<\/p>\r\n

                                      Virus UPSC PYQs<\/span><\/h2>\r\n

                                      Q1:<\/strong> Consider the following statements: (UPSC Prelims 2025)<\/strong><\/span><\/p>\r\n

                                        \r\n\t
                                      1. No <\/span>virus can survive in ocean waters.<\/span><\/li>\r\n\t
                                      2. No <\/span>virus can infect bacteria.<\/span><\/li>\r\n\t
                                      3. No virus can change the cellular transcriptional activity in host cells.<\/li>\r\n<\/ol>\r\n

                                        How many of the statements given above are correct?<\/span><\/p>\r\n

                                        a) Only one<\/span><\/p>\r\n

                                        b) Only two<\/span><\/p>\r\n

                                        c) All three<\/span><\/p>\r\n

                                        d) None<\/span><\/p>\r\n

                                        Ans: (d)<\/strong><\/p>","protected":false},"excerpt":{"rendered":"

                                        An overview of virus structure, taxonomy, and major groups including adenovirus, retrovirus, herpesvirus and examples like influenza, HIV, and smallpox.<\/p>\n","protected":false},"author":6,"featured_media":4018,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[173],"tags":[40,539],"class_list":{"0":"post-4017","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-quest-level-3","8":"tag-quest","9":"tag-virus"},"acf":[],"_links":{"self":[{"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/posts\/4017","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/comments?post=4017"}],"version-history":[{"count":1,"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/posts\/4017\/revisions"}],"predecessor-version":[{"id":19984,"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/posts\/4017\/revisions\/19984"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/media\/4018"}],"wp:attachment":[{"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/media?parent=4017"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/categories?post=4017"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/vajiramandravi.com\/upsc-exam\/wp-json\/wp\/v2\/tags?post=4017"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}