Diseases like plant mosaic, leafroll, tomato bushist, rice tungro, papaya ring spot, banana bunchy top etc. are caused by virus. About 300 plant diseases are caused by virus and reduce yield. Dr. Siddiq Publications

Virus cause serious diseases like human measles, measles, polio, rabies, influenza, jaundice, herpes, dengue, hepatitis, Ebola, AIDS, Sars, swine flu, cancer etc. Virus that cause cancer in the human body are called oncogenic virus. Cancer-causing DNA viruses are called simian and RNA virus are called retrovirus. Human papillary virus (HPV) causes tumors, Kaposi sarcoma herpes virus (KSHV) causes skin cancer and hepatitis-C virus causes liver cirrhosis.

1. Causing human diseases: Virus cause serious diseases like human measles, measles, polio, rabies, influenza, jaundice, herpes, dengue, hepatitis, Ebola, AIDS, Sars, swine flu, cancer etc. Virus that cause cancer in the human body are called oncogenic virus. Cancer-causing DNA viruses are called simian and RNA virus are called retrovirus. Human papillary virus (HPV) causes tumors, Kaposi sarcoma herpes virus (KSHV) causes skin cancer and hepatitis-C virus causes liver cirrhosis.
2. Plant Diseases: Diseases like plant mosaic, leafroll, tomato bushist, rice tungro, papaya ring spot, banana bunchy top etc. are caused by virus. About 300 plant diseases are caused by virus and reduce yield. Dr. Siddiq Publications
3. Cattle Diseases: Bovine Spring Disease; Foot and mouth disease in cattle, buffalo, sheep, goats, pigs, etc. and rabies in dogs and cats are caused by virus.
4. Poultry Diseases: Poultry diseases such as avian, influenza, bird flu, swine flu etc. are caused by virus.
5. Silkworm disease: Nuclear polyhedrosis virus (NPV) causes silkworm disease. This resulted in a disaster in the silk industry.
6. Destroying Beneficial Bacteria: Virus destroy many of our beneficial bacteria. Dr. Siddiq Publications
7. Loss of soil fertility: Cyanophage virus destroys nitrogen-fixing blue-green algae. As a result, soil fertility decreases.
8. Industrial: Alcohol destroys industrially used spirits. This caused an industrial disaster.
9. Emerging virus: Virus that cause disease in new host species from the original host are called emerging virus. Birds are the primary host of bird flu and influenza virus and monkeys are the host of HIV. They later caused disease in the human body. In 1918-1919, more than 21 million people died of bird flu in the world. Emerging virus like HIV, SARS, Nile, Ebola etc.
10. Zoonetic Virus: Virus that are naturally transmitted from vertebrates to humans are called zoonetic viruses. Zoonetic virus are:
(i) SARS Corona virus: Severe acute respiratory syndrome caused by coronavirus is called SARS. The disease is transmitted from bats to humans. In 2003, about 800 people died in China, Taiwan, Canada and other countries due to Sars disease.
(ii) MERS Corona Virus: The disease caused by Middle East respiratory syndrome coronavirus is called MERS. The disease is transmitted from bats to humans. In 2003, Mars disease killed many people in the Middle East.
(iii) Sars Corona Virus-2: The disease caused by Severe acute respiratory syndrome coronavirus-2 is called SARS-2. The disease is globally known as COVID-19. It has been transmitted to humans from bats in Yunnan, China. Six million people died from the disease from 2019-2023.
11. Pandemic Virus: Numerous virus have become epidemic and spread rapidly among people. Virus causing worldwide epidemics are-
(i) HIV : The virus that causes the deadly disease AIDS is HIV. It destroys human white blood cells. The body’s immune system is reduced or destroyed. In Bangladesh, the number of AIDS patients and the number of deaths are increasing.
(ii) Ebola virus: Ebola is a deadly virus. In 1976, a farmer died of the virus on the banks of the Ebola River in Congo, Africa. The dead farmer’s eyes, nose, ears and throat were bleeding. The virus was named Ebola virus after the river. An outbreak of the Ebola virus in Zaire, Africa. In 2014-15, the Ebola outbreak killed 11,000 people, including 250 health workers, in Guinea, Sierra Leone and Liberia. Disease symptoms appear within 2-21 days.
(iii) Bird flu virus: Bird flu disease is caused by Avian Influenza virus (Hemagglutinin-1 Neuraminidase type-1=H1N1). In 2008, bird flu became an epidemic in Bangladesh and West Bengal. The disease affected poultry and destroyed the poultry industry. Dr. Siddiq Publications
(iv) Swine flu virus: Swine flu is caused by Swine Influenza virus. Swine flu is caused by influenza viruses H5N1 and H1N1. Swine flu was detected in April 2009. Humans and pigs are infected by this virus. Thousands of chickens die from this disease every year in Bangladesh. In India, 34,000 people were infected in February 2015.
(v) Zika virus: The disease caused by Zika virus is called Zika virus. In 1947, the Zika virus was first detected in rhesus monkeys living in the Zika forest in Uganda. It is spread by mosquitoes. In March 2016, Zika virus was detected in Chittagong district of Bangladesh. Zika virus carrier Aedes aegypti. Aedes albopictus mosquito.
(vi) Nipah virus: The first outbreak of Nipah virus occurred in pig farms in Malaysia in 1999. Bats are carriers of this virus. It is transmitted to the human body through the raw juice of dates. It is an RNA virus. The attack of this virus causes complications in breathing. Dr. Siddiq Publications
(vii) Chikungunya: Chikungunya is an α-virus (RNA). Its carriers are Aedes aegypti and Aedes albopictus mosquitoes. Chikungunya was first discovered in 1952 in Tanzania, Africa. It was first caught in Bangladesh in 2008. Chikungunya spread widely in Bangladesh in April-May 2017. Chikungunya is also known as Langra Zor.
12. Destruction of Beneficial Microorganisms: Some virus cause economic losses by destroying beneficial microorganisms. Xymophage virus destroys yeast used in the wine or alcohol industry. Cyanophage viruses destroy blue-green algae. It reduces the amount of nitrogen in the soil. T2 bacteriophage kills E. coli bacteria that live in the human gut.

1. Vaccines: Vaccines are made from virus to prevent certain viral diseases. Vaccines for smallpox, polio, dropsy, jaundice, cholera, plague, blood dysentery, typhoid etc. are made from virus.
2. As medicine: Some phage virus are used in cholera, typhoid, blood dysentery and plague.
3. Destruction of Harmful Bacteria: Virus benefit us by destroying harmful bacteria present in the environment. Faze virus destroys E. coli bacteria.
4. Control of Harmful Insects: Control of harmful insects has become possible with the help of virus. Nuclear polyhydrosis virus is used as an insecticide in the United States. Dr. Siddiq Publications
5. Research: Virus are currently being used in genetic and molecular biology research. Virus are used as vectors in genetic engineering.
6. Enhancement of flower beauty: White spots are formed on tulip flowers due to virus attack. It increases the beauty of flowers. It is called Broken Tulip. The market value of broken tulip flowers is high.
7. In biological research: virus are widely used in microbiology, medical science and genetic engineering research.
8. Bacteriophage therapy: Phage virus are used to destroy bacteria causing diseases such as typhoid, cholera, dysentery, plague etc.
9. Balancing marine ecosystems: Phage virus destroy marine bacteria and release huge amounts of CO2. It maintains the tectonic balance of the ocean. One milliliter of sea water contains one lakh viruses.
10. Biological Control: Currently virus are widely used in biological control worldwide. Myxoviruses are being used to control rabbits in Australia.
11. Evidence for Evolution: Virus are the greatest reservoir of biodiversity. It bears the characteristics of the living and the inanimate. Therefore, virus play an important role in uncovering information about the origin and evolution of organisms.

The process by which the DNA of the virus combines with the DNA of the host bacteria to form a replicase and remain inside the host cell is called the lysogenic cycle. A virus that exhibits a lysogenic cycle is called a temperate phage. The lysogenic cycle occurs in lambda (λ) phage (P1, M13) viruses. The steps of this cycle are discussed.
1. Infection stage: Lambda (λ) phage is attached to the cell wall of E. coli bacteria. The virus attaches to the receptive spot of the bacterium by tactile fibers. Then the tangential fibers contract. The base plate of the virus is in contact with the bacterial cell wall. The lysozyme enzyme is secreted from the tail of the virus. Lysozyme dissolves mucopeptides in the bacterial cell wall. As a result, a fine pore or duct is formed. This process is called drilling. Virus DNA enters the bacterial cell through pores or tubules. But the capsid coat remains outside.
2. Entry phase of phage DNA into host cell: Viral DNA is transferred from head to tail. The viral DNA then enters through pores or tubules in the cell wall of the host bacteria. But the capsid coat remains outside the host cell.
3. Prophage formation: As soon as the viral DNA enters the host bacteria, the genes of the host cell become activated. Because of this protein and lysozyme cannot be produced. As a result, the capsid envelope surrounding the viral DNA cannot be formed. Endonuclease enzymes cut a specific segment of bacterial DNA. The integrase enzyme joins the viral DNA to the cut in the bacterial DNA. Attached virus DNA is called prophage.
4. Increase in the number of prophages: Prophages attach to the bacterial genome and divide repeatedly and are transmitted to the bacterial cell. Such phage is called lysogenic phage or temperate phage. It can enter the lytic cycle.
Lysogenic virus can infect humans and other animals. For example, Herpes simplex virus. Dr. Siddiq Publications

Virus multiply inside host cells. A great stress is created in the host’s cell wall. The mucoprotein complex of the host cell wall is damaged by the action of the enzyme lysozyme. As a result, the cell wall ruptures and T2 phase is released. The breaking down of the cell wall is called lysis. Fungi of such nature are called lytic fungi or virulent fungi.

Virus DNA remains inactive for 12-22 minutes after entering the host cell. During this time the DNA of the virus takes control of the genetic activity of the host.

T2 bacteriophage attaches to the cell wall of E. coli bacteria. The virus attaches to the receptive spot of the bacterium by tactile fibers. Then the tangential fibers contract. The base plate of the virus comes into contact with the cell wall of the bacteria. An enzyme called lysozyme is released from the tail of the virus. Lysozyme dissolves mucopeptides in the bacterial cell wall. As a result, a fine pore or duct is formed. This process is called drilling. Virus DNA enters the bacterial cell through pores or tubules. But the capsid coat remains outside.

The process by which virus complete their life cycle by invading and destroying bacterial cells is called the lytic life cycle. The stages of the lytic cycle of T2 phage or virulent phage are discussed.
1. Infection stage: T2 bacteriophage attaches to the cell wall of E. coli bacteria. The virus attaches to the receptive spot of the bacterium by tactile fibers. Then the tangential fibers contract. The base plate of the virus comes into contact with the cell wall of the bacteria. An enzyme called lysozyme is released from the tail of the virus. Lysozyme dissolves mucopeptides in the bacterial cell wall. As a result, a fine pore or duct is formed. This process is called drilling. Virus DNA enters the bacterial cell through pores or tubules. But the capsid coat remains outside.
2. Dormant period: Virus DNA remains inactive for 12-22 minutes after entering the host cell. During this time the DNA of the virus takes control of the genetic activity of the host.
3. Production of phage organelles: After virus DNA enters the host cell, it establishes dominance over host DNA. Then, with the help of polymerase enzymes, it uses the host nucleotides to produce new DNA. The new DNA produced makes RNA. RNA makes proteins with the help of host ribosomes. The protein molecules join together to form the capsid. Note that separate proteins are produced to form the head, tail and tangential filaments of the phage. Dr. Siddiq Publications
4. Formation of T2 Phase: Protein molecules join together to form head, tail and tangential fibers. Later these three parts join together to form the protein shell. A copy of the DNA molecule enters the brain. The tail and other parts are then joined to form the T2 phase. In this way, the bacterial cell produces numerous false virus. Finally, the synthesis of lysozyme enzyme occurs in the tail of the virus.
5. Lysis: Virus multiply inside host cells. A great stress is created in the host’s cell wall. The mucoprotein complex of the host cell wall is damaged by the action of the enzyme lysozyme. As a result, the cell wall ruptures and T2 phase is released. The breaking down of the cell wall is called lysis. Fungi of such nature are called lytic fungi or virulent fungi.
The period from when the host infects the bacterium to the formation of a full-fledged virus is called eclipse. T2 phage takes 30 minutes for virus to multiply (eclipse). During this time 300 T2 phases are generated per cell. The released new phage then invades the new host cell. Dr. Siddiq Publications

HIV is a round virus. It consists of two parts. Nucleic Acids and Capsids.
1. Nucleic Acid: At the center of the virus is a core of nucleic acid. It is called nucleoid. It is made up of RNA. At its core is reverse transcriptase. Reverse transcriptase converts RNA into DNA. It carries the genetic characteristics of the virus and plays a role in the synthesis of new viruses.
2. Capsid: The outer coat of nucleic acid is called capsid. Capsids can be spherical or polygonal. It is made up of numerous protein molecules. Each protein molecule is called a capsomer. Outside the capsid is a 1100 µm thick phospholipid bilayer. Phospholipids consist of two types of glycoproteins. GP-120 and GP-41. GP-120 helps viruses attach to host cells. GP-120 is chemically related to GP-41.