What is a Virus?

By Dr Ananya Mandal, MD
Viruses are tiny organisms that may lead to mild to severe illnesses in humans, animals and plants. This may include flu or a cold to something more life threatening like HIV/AIDS.

How big are viruses?

The virus particles are 100 times smaller than a single bacteria cell. The bacterial cell alone is more than 10 times smaller than a human cell and a human cell is 10 times smaller than the diameter of a single human hair.

Are viruses alive?

Viruses by themselves are not alive. They cannot grow or multiply on their own and need to enter a human or animal cell and take over the cell to help them multiply. These viruses may also infect bacterial cells.
The virus particle or the virions attack the cell and take over its machinery to carry out their own life processes of multiplication and growth. An infected cell will produce viral particles instead of its usual products.

Structure of a virus

A virion (virus particle) has three main parts:

• Nucleic acid – this is the core of the virus with the DNA or RNA (deoxyribonucleic acid and ribonucleic acid respectively). The DNA or RNA holds all of the information for the virus and that makes it unique and helps it multiply.
• Protein Coat (capsid) – This is covering over the nucleic acid that protects it.

Lipid membrane (envelope) – this covers the capsid. Many viruses do not have this envelope and are called naked viruses.

Receptors

Viruses are not simply taken into cells. They must first attach to a receptor on the cell surface. Each virus has its specific receptor, usually a vital component of the cell surface. It is the distribution of these receptor molecules on host cells that determines the cell-preference of viruses. For example, the cold and flu virus prefers the mucus lining cells of the lungs and the airways.

How do viruses infect?

Viruses do not have the chemical machinery needed to survive on their own. They, thus seek out host cells in which they can multiply. These viruses enter the body from the environment or other individuals from soil to water to air via nose, mouth, or any breaks in the skin and seek a cell to infect.
A cold or flu virus for example will target cells that line the respiratory (i.e. the lungs) or digestive (i.e. the stomach) tracts. The HIV (human immunodeficiency virus) that causes AIDS attacks the T-cells (a type of white blood cell that fights infection and disease) of the immune system.

Life cycle of a basic virus

There are a few basic steps that all infecting viruses follow and these are called the lytic cycle. These include:

1. A virus particle attaches to a host cell. This is called the process of adsorption
2. The particle injects its DNA or RNA into the host cell called entry.
3. The invading DNA or RNA takes over the cell and recruits the host’s enzymes
4. The cellular enzymes start making new virus particles called replication
5. The particles of the virus created by the cell come together to form new viruses. This is called assembly
6. The newly formed viruses kill the cell so that they may break free and search for a new host cell. This is called release.

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