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Viruses exact an enormous toll on the human population and are the single most important cause of infectious disease morbidity and mortality in the world. Viruses are microscopic organisms that can infect animals, plants, fungi, and even bacteria. A virus is not technically alive because it cannot function unless it has a host. On their own, viruses are just little packages of genetic material inside a protein shell called a capsid. Their favorite and only activity is to hang around in an ‘off’ position and wait. 

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Viral Anatomy



The viral structure comprises three main components – Nucleic acid - found within its inner core that contains the genetic information for the synthesis of proteins and 


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replication. The Capsid - a protein layer or covering that forms a shell enclosing the genetic material of the virus, and the Envelope or glycoprotein coat- lipoprotein bilayer that encloses the capsid. 


A virus in its infectious state is called a virion. In this state, certain infectious proteins are present on its surface which enhances the infection process. The infection cycle consists broadly of five steps:  

How Does Infection Occur


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  1. Attachment: The viral particle attaches to the host cells using its outer layer or envelope that displays a variety of proteins complementary to host cell receptors. 

  2. Entry: The entry of the viral DNA into the host cell using cell membrane fusion using proteins. 

  3. Genome replication and protein synthesis: The replication of the viral genome and synthesis of viral capsid proteins. 

  4. Assembly: The viral progeny is assembled using the newly synthesized genetic material and viral proteins. 

  5. Release: Newly assembled viral progeny exits the cell and infects other cells, thus re-starting the infection cycle. This process continuously repeats itself, rendering the host more and more symptomatic.  

Viral epidemiology



Scientific approaches to the study of viruses and viral disease began in the 19th century leading to the identification of specific diseases caused by viruses.  Clinical observations followed that enabled the identification of, and further differentiation of; several viral diseases (e.g., smallpox vs. Chickenpox and measles vs. Rubella) The progress in defining the pathology of many viral diseases; the understanding of disease at the level of cells and tissue, further led to advances in the systematic use of animal subjects for studies of the pathogenesis of infectious diseases, including those caused by viruses.  Few viruses change the function of the cells instead of killing the cells. For example, Human Papillomavirus, Epstein Barr Virus causes uncontrolled replication of cells that leads to cancer.  

There are numerous viruses that are transmitted between humans and that are significant causes of illness and death. Seasonal influenza viruses, for example, circulate globally every year, causing illness in tens of millions of people worldwide; an estimated 290,000 to nearly 650,000 people die from seasonal influenza each year.  In addition, new types of infectious viruses emerge periodically. In many instances, these viruses “jump” to humans from an animal reservoir,

There are a million virus particles per milliliter of seawater – for a global total of 1030 virions! Lined up end to end, they would stretch 200 million light-years into space. 

such as bats, pigs, or primates; this occurs when a human is in close contact with an animal that carries the virus. Zoonoses are infectious diseases caused by infectious agents (viruses, bacteria, parasites, prions) that are passed between animals and people. Often the virus then evolves to become transmissible between humans. Examples of infectious viruses that originated from animal reservoirs in the mid-20th or early 21st century and went on to cause epidemics or pandemics of disease in humans include ebolaviruses, SARS coronavirus, influenza A H1N1, human immunodeficiency virus (HIV/AIDS), and more recently;  SARS-CoV-2 coronavirus.


Genomic and epidemiological monitoring has become an integral part of our response to emerging and ongoing epidemics of viral infectious diseases. Advances in high-throughput sequencing, including portable genomic sequencing at reduced costs and turnaround time, are paralleled by continuing developments in methodology to infer evolutionary histories (dynamics/patterns) and to identify factors driving viral spread in space and time. The traditionally static nature of visualizing phylogenetic trees that represent these evolutionary relationships/processes has also evolved, albeit perhaps at a slower rate. Advanced visualization tools with increased resolution assist in drawing conclusions from phylogenetic estimates and may even have the potential to better inform public health and treatment decisions 

Perhaps an even more exciting development in vaccine technological advancements is the means to introduce genetic instructional material -Messenger ribonucleic acid (mRNA), into the body.  

DNA (deoxyribonucleic acid) is a double-stranded molecule that stores the genetic instructions your body's cells need to make proteins.  

Proteins, on the other hand, are the ‘workhorses’ of the body. Nearly every function in the human body – both normal and disease-related – is carried out by one or many proteins.  mRNA is just as critical as DNA. Without mRNA, your genetic code would never get used by your body. Proteins would never get made. And your body would not perform its functions. mRNA plays a vital role in human biology, specifically in a process known as protein synthesis. mRNA is a single-stranded molecule that carries genetic code from DNA in a cell’s nucleus to ribosomes, the cell’s protein-making machinery.

Vaccines traditionally contain either weakened viruses or purified signature proteins of the virus. An mRNA vaccine is different because rather than having the viral protein injected,

Viruses evolve faster than any other living organism. When one copy of the virus genome gets into a host cell, it multiplies incredibly quickly.  Within hours, thousands of copies can be made from a single virus.  Since viruses cycle through multiple generations so quickly, they end up making frequent mistakes (mutations) when copying their genetic information. Mutations help viruses to circumvent medications and elude host cell defenses meant to destroy them.

a person receives genetic material – mRNA – that encodes the viral protein. When these genetic instructions are injected into the upper arm, the muscle cells translate them to make the viral protein directly in the body.


This approach mimics what the SARS-CoV-2 does in nature – but the vaccine mRNA codes only for the critical fragment of the viral protein –spike protein. This gives the immune system a preview of what the real virus looks like without causing disease. This preview gives the immune system time to design powerful antibodies that can neutralize the real virus if the individual is ever infected.


These powerful new technologies/ techniques are leading to breakthroughs in foundational problems in viral pathogenesis, such as the nature of virus-cell interactions that produce disease, immunoprotective and immunopathologic host responses to infection, and viral and host determinants of contagion.  

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