The nature of viruses as an infectious agent

The name virus was coined from the Latin word meaning slimy liquid or poison.
In the early years of discovery, viruses were referred to as filterable agents. Only later was the term virus restricted to filterable agents that require a living host for propagation.

Almost every ecosystem on Earth contains viruses. Viruses were certainly around long before humans found the first plant virus (Tobacco mosaic virus) in 1892, the first animal virus (Foot-and-mouth disease virus) in 1898 and the first human virus (Yellow fever virus) in 1901.

The first clue to the nature of viruses was in 1898, by Friedrich Loeffler and Paul Frosch. Viruses are living organisms that cannot replicate without a host cell.

Virus forms and sizes

Most viruses that have been studied have a diameter between 20 and 300 nanometres.

The criteria used for classifying viruses into families and genera (singular: genus) are primarily based on three structural considerations:

  1. The type and size of their nucleic acid.
    All viruses contain nucleic acid, either DNA or RNA (but not both).
  2. The shape and size of the capsids. A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers.
    A helical virus is a virus that has a capsid shaped in a filamentous or rod-shaped structure that has a central cavity that encloses its nucleic acid.
    An icosahedral virus is a virus consisting of identical subunits that make up equilateral triangles that are in turn arranged in a symmetrical fashion.
    Envelope Viruses on the other hand cover themselves with a modified section of cell membrane, creating a protective lipid envelope.
  3. The presence of a lipid envelope, derived from the host cell, surrounding the viral nucleocapsid.

While in this form, outside the cell, the virus is metabollically inert (i.e lacking the ability or strength to move or chemically inactive).

Before entering a cell, viruses exist in a form known as virions. The object on which virus live on for some time before it enters a cell is known as a fomite.

Virus phases of infection

When it comes into contact with a host cell, a virus can insert its genetic material into its host, literally taking over the host’s functions.

An infected host cell produces more viral protein and genetic material instead of its usual products. These proteins are responsible for altering normal cellular functions which in some cases allow the infected cell to evade the immune system.

There are 3 major virus infection phases which are:

  1. Lytic infection
    This is a process where new viruses are formed, self-assemble, and burst out of the host cell, killing the cell and going on to infect other cells.
  2. Latent infections
    Some viruses may remain dormant inside host cells for long periods, causing no obvious change in their host cells (a stage known as the lysogenic phase).
  3. Persistent infection. This is when the virus is capable of replicating slowly, silently or at low levels without causing excessive damage to the host cell.

Measles virus, HIV- 1 and Hepatitis B virus are examples of persistent infection.

When a dormant virus is stimulated (by external or cellular factors), it enters the lytic phase

When you get a virus, you may not always get sick from it. Your immune system may be able to fight it off.
But if your local immunity is compromised by stimuli such as fever, menstruation, exposure to sunlight, you may be infected or dormant viruses in you may be activated.

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