What is a Virus?: Structure & Causes

What is a Virus



A virus is a microscopic organism that can replicate only inside the cells of a host organism. Most viruses are so tiny they are only observable with at least a conventional optical microscope. Viruses infect all types of organisms, including animals and plants, as well as bacteria and archaea. Approximately 5000 different viruses have been described in detail at the current time, although it is known that there are millions of distinct types. Viruses are found in virtually every ecosystem on Earth, and these minute life forms are thought to be the most abundant type of biological entity. The study of viruses is known as virology, a specialty within the field of  microbiology.

Almost every ecosystem on Earth contains viruses; they are considered the most abundant biological entity on the planet.

Before entering a cell, viruses exist in a form known as virions. During this phase they are roughly one-hundredth the size of a bacterium and consist of two or three distinct parts:

  • Genetic material – either DNA or RNA
  • Protein coat – called the capsid, this protects the genetic information
  • Lipid envelope – sometimes present around the protein coat when the virus is outside of the cell

Viruses are unique microorganisms because they cannot reproduce without a host cell. After contacting a host cell, a virus will insert genetic material into the host and take over that host’s functions.

The cell, now infected, continues to reproduce, but it reproduces more viral protein and genetic material instead of the usual cellular products. It is this process that earns viruses the classification of “parasite.”

Viruses come in a wide array of shapes and sizes and are often fitted into the following categories:

  • Helical – for instance, the tobacco mosaic virus.
  • Icosahedral – these near-spherical viruses make up the majority of animal viruses.
  • Envelope – some viruses cover themselves with a modified section of cell membrane, creating a protective lipid envelope. These include the influenza virus and HIV.
  • Complex – a non-standard shape that can include both helical and icosahedral forms.

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.

    How are viruses spread?

    Viruses may spread vertically (from mother to child) or horizontally (from person to person). A virus’ ability to spread depends on the makeup of the virus.

    Some viruses can spread by simple contact, exchanges of saliva, coughing, or sneezing. Some require sexual contact, while others go through the fecal-oral route via contaminated food or water. Still other viruses require an insect like a mosquito to carry the virus from person to person.

    What diseases are caused by viruses?

    Several human diseases are caused by viruses; these include:

    • Smallpox
    • The common cold
    • Measles
    • Chickenpox
    • Hepatitis
    • Influenza
    • Human papilloma virus
    • Shingles
    • Herpes
    • Polio
    • Rabies
    • Ebola
    • Hanta fever
    • HIV (the virus that causes AIDS)
    • Cold sores
    • SARS (Severe acute respiratory syndrome)
    • Dengue
    • Epstein-Barr virus
    • Some types of cancer

    How do we fight viruses?

    When the body’s immune system detects a viral infection, it begins to respond. A process called RNA interference is started, this process degrades viral genetic material and enables cells to survive the infection.

    The immune system also produces specific antibodies that are capable of binding to viruses, making them non-infectious. In addition, the body’s T cells are sent to destroy the virus.

    Although most viral infections result in a protective response from the immune system, viruses such as HIV specialize in evading the immune system using a number of different techniques.

    Neurotropic viruses are also very capable of avoiding our natural immune system’s response to infection. These viruses are capable of infecting nerve cells and are responsible for diseases that include polio, rabies, mumps, and measles.

    How are viruses prevented and treated? <– read more