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The multiplication of virus cells causes basic medical symptoms like common cold, flu and fever.

Tobacco mosaic virus is the name of the first discovered virus by two scientists, namely Ivanoski and Beijerinck.

Ebola, marburg, hantavirus, lassa, smallpox, rabies, influenza and dengue are considered to be the deadliest viruses in the world.

Even though there are many unknown viruses, there are approximately 320,000 types of viruses that attack mammals.

When the virus enters the host, they invade your living cells and use those cells to produce other virus cells. The multiplication of virus cells causes basic medical symptoms like common cold, flu and fever.

Introduction Virus is a Latin word which means “poison” or “slimy liquid”. Just like its meaning, viruses are small infectious agents that are capable of multiplying in living cells of plants, animals and bacteria. A virus is made of DNA or RNA genome, inside a protein shell known as a capsid. They cannot survive or reproduce outside the body of the host. Viruses are popularly known for being the cause of a contagion. A virus particle is made of genetic material stored inside a protein shell or a capsid. The genetic material present in the virus might have single-stranded or double-stranded DNA or RNA, which might be linear or circular in shape. Usually, viruses measure in width from 20 nanometers (nm) to 400 nm. Reproduction in viruses occurs when they infect their host cells and convert them into virus-making factories. There are many other interesting properties of a virus, based on their size and shape. Now, let’s look at the life cycle of a virus to get a better understanding.

The stages in the life cycle of a virus are mentioned below: 1) Attachment or Absorption Here, the attachment proteins on the surface of the virus align to specific receptors on the surface of the animal cells. Apart from virus binding, cellular receptors usually have glycolipids or glycoprotein. The interaction between these specific attached proteins and cellular receptors determine the host range. Host range is crucial in viral attachment, as the attachment of a virus is, specific binding between capsid proteins and specific receptors on the surface of the host cell. If the host range is narrow, the virus can only infect a small number of cell types. Similarly, if the host range is broad, the virus can infect a large number of cell types. 2) Penetration or Entry In this stage, the virus or its genetic material enters the cell. Viruses with envelopes usually enter through fusion with the membrane. Sometimes viruses take the cells in bulk by a bulk transport process known as endocytosis. Some viruses inject their DNA into the cell. 3) Genome Replication and Gene Expression Viral genome either has double or single-stranded molecules of DNA or RNA, but never both together. In this stage, the viral genome is copied and its genes are expressed to make viral proteins. This way, new virus particles can be assembled. The genetic material for this process is from the host; the tools for replication and gene expression are also given by the host. The produced viral proteins are different and vary from one virus to the other. All viruses should encode capsid proteins and all enveloped viruses should encode envelope proteins. Sometimes viruses also encode proteins that hinder the host genome, by blocking the host’s defence techniques, to benefit the virus. Viruses encode proteins that hinder the host genome, aid in viral replication and have a major role in the life cycle of viruses. 4) Assembly Capsomers are the outer covering of proteins that protect the genetic information of a virus. In this stage, newly developed capsid proteins come together to form capsomers. Capsomers interact with other capsomers to form a fully developed capsid protein. Viruses such as the head-tail viruses, first assemble an empty capsid and then store it with a viral genome. But, the rest of the viruses create the capsid around the viral genome. 5) Release This is the last stage in the life cycle of viruses, where they release newly created viruses from the host cell. Different kinds of viruses exit the cell in different methods. Some follow the process called lysis, where the virus bursts the host cell. The other viruses follow the process called exocytosis, where the virus exits from the cell’s own pathways. There are some other viruses which bud from the plasma membrane of the cell. When the new virus is released, it has the ability to kill the host cell. But some other viruses do not hinder the host cell, leave it as it is and continue to make more virus particles.

A viral infection occurs when the virus uses the host’s cell to make copies of itself. Viruses are highly diverse. They come in all shapes and structures and depending on their species, the virus attacks different hosts. If the virus is void of a host, then it is considered non-living. The virus is only considered living when it is inside a host.  

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