Understanding viruses

SARS - a new and dangerous disease has infected many in South-east Asia and taken the lives of several people. It originated in Guandong, China in November and has killed an estimated 110 people and infected more than 3,000 in total. The SARS or Severe Acute, Respiratory Syndrome is a respiratory illness and a form of pneumonia, caused by the Coronavirus. SARS is marked by a high fever, dry cough and other flu-like symptoms but it progresses into pneumonia.

 

The coronavirus is related to the many viruses that cause common cold. SARS is transmitted, like any cold, through droplets transmission, when a person infected with SARS sneezes or coughs and the other person breathes in these droplets. But what really are viruses and how do they infect the body?

 

The virus
The most common virus is the one, which causes colds or flu, the symptoms are fever, congestion, sore throat and cough among others. The common cold does not have a cure yet, because there are hundreds of viruses, which can cause it. Viruses are responsible for other serious diseases too like AIDS, Ebola and cancer apart from other infectious diseases like hepatitis and herpes.

Viruses are parasites; they have a noncellular structure consisting of DNA or RNA and a protein coat. As they are extremely small (20-300nm), they cannot be seen through a light microscope, they require an electron microscope.

 

Viruses were discovered around the end of the 19th century by a Russian botanist, Dimitri Iwanowski in tobacco plants; in 1900 a similarly filterable agent (found in the tobacco plant) was reported for foot-and-mouth disease of cattle. In 1935, American virologist W. M. Stanley crystallized tobacco mosaic virus for which he shared the 1946 Nobel Prize in Chemistry with J. H. Northrup and J. B. Summer.

Viruses are generally classified according to properties such as size, the type of nucleic acid they contain, the number of protein they contain, host species and immunological characteristics.

 

The virus structure
A virus particle consists of: a) Nucleic acid - set of genetic instructions, either DNA or RNA, either single-stranded or double-stranded; b) A coat protein, which surround the DNA or RNA to protect it and c) A lipid membrane, which in turn surrounds the protein coat (these are also called enveloped viruses as opposed to naked viruses; these are however found in only some viruses, including influenza)

Viruses vary greatly in their shape and complexity, some are round and look like popcorn while others have complicated shapes resembling spiders.

 

A virus in your body
The principle difference between virus and so called 'living cells', lies in the fact that viruses do not contain enzymes, which are needed to carry out chemical reaction for life. So, a virus needs a host cell, either a bacteria, plant or animal in order to live and make more viruses. Viruses cannot function outside a host cell.

Viruses are in the environment all the time, waiting to find a host. They can enter through the nose, mouth, or through a break in the skin and once inside, they look for a host cell to infect. Once inside the cell, viruses go through what is called a lyctic cycle; during this cycle, the viral enzymes take over the enzymes of the host cell and start reproducing themselves via the host cell machinery. This is how a lyctic cycle runs:

A virus particle attaches itself to the host cell

The particle releases genetic instruction to the host

The injected genetic material then recruits the host cell's enzymes and produce more virus particles

The new particles then form into new viruses

The new viruses then break free from the host cell.

 

A cold virus generally attack cells that line the respiratory or even digestive tracks; the Human Immunodeficiency Virus (HIV), which causes AIDS, attacks the T-cells of the immune system. Viruses do not actually kill the host cells, instead they persist within them in one form or another. Viruses can live for years in a person, without the person showing any outward symptoms. For example a person infected with HIV can live for years without manifesting any of the symptoms. In the case of cancer, the viruses are known as retroviruses. These viruses' genetic material is RNA, but however they carry an enzyme that copies RNA information into DNA molecules. This is then integrated in the host cell and can reside inside and start producing more viruses via the host cell machinery.

 

Viral diseases
There are several types of viruses causing a host diseases in humans, animals and plants. Sometimes diseases are caused by the body's reaction to the virus infection. Viruses cause a large number of illnesses in human, they include: the common cold, influenza, gastroenteritis, measles, mumps, chickenpox, rubella, glandular fever, hepatitis, AIDS, cancer and herpes.

Antiviral drugs are effective for several viral infections, while immunizations have been developed for viral infections such as measles, mumps, rubella and polio.

 

The Coronavirus
Coronaviruses have a corona-like, (halo or crown-like) appearance when viewed under a microscope. These viruses cause of mild to moderate upper-respiratory illness in humans and are associated with respiratory, gastrointestinal, liver and neurologic disease in animals. Coronaviruses can survive in the environment for as long as three hours. Medical authorities do not yet have enough information in order to determine the full range of illness that it might cause. Coronaviruses have earlier been linked to pneumonia in humans, generally with people who have weakened immune systems. The viruses is also known to cause disease in animals, including cats, dogs, pigs, mice, and birds.