History and Discovery of Viruses

History and Discovery of Viruses

Virus can be defined as "non-cellular infectious particle which contains either RNA or DNA, normally enclosed in a proteinaceous coat and reproduces only in living cells". The word virus is derived from Latin word Venom. Venom means poisonous fluid. Viruses use the biosynthetic systems of their hosts for their own synthesis. They are then transferred to other cell.

Virus was called poison during the time of Louis Pasteur (1822- 1895) and Robert Koch (1843 1910). The viruses caused disease and death. The present concept of virus has entirely charged. Now viruses are described as particles of nucleic acid with a protein coat. They replicate in the cell and cause many diseases like influenza, hepatitis (edge), small pox and AIDS. The branch of biology which deals with the study of virus is known as Virology.

1. Discovery of Vaccine against Virus

Viruses cause many diseases. First vaccine was discovered against virus. Edward Jenner vaccinated an 8 year boy in 1796. Cowpox is a viral disease of cow. The cow develops lesion on its skin. Material (pus) was removed from this lesion on the hand of milkmaid. Jenner used this pus ( + for  vaccination of the boy. After 6 weeks, Jenner removed pus from the small pox patient. He injected this pus into the boy. The boy did not develop disease. So the boy had been successfully vaccinated. Later, Jenner used material from cowpox lesion for vaccination. He successfully vaccinated 23 persons. Cow is called vacca in Latin. As the material for vaccination was obtained from cow or vacca, so Louis Pasteur used the term Vaccination. 

2. Work of Charles Chamberland

He was an associate of Pasteur. In 1884, he found that the bacteria cannot pass through porcelain filters. While Virus causing rabies can pass through these filters. Rabies is a human disease. Rabies is transferred to human by the biting of rabid dog, fox, cats, bats and other animals. In those days, people called virus a toxic substance as it caused diseases. They described these unseen diseases causing agent as filterable viruses.

3. Work of Ivanowski

lanowski found in 1892 that the viruses of tobacco mosaic disease are filterable. He grounded the infected plant. He took bacteria free filtrate from it. He placed this filtrate on the healthy leaves of tobacco. This filtrate produced disease in the healthy plant Later, it was discovered that similar filterable ultramicroscopic viruses also caused foot and mouth disease (1898) and yellow fever (1901) in animals.

4. Discovery of Bacteriophage

Bacteriophage or phage virus is a virus that infects bacteria. It was independently discovered by Twort in 1915 and D'Herelle in 1917. Twort observed that sometimes, bacteria undergo lysis. During lysis, bacteria break, dissolve and disappear. This lysis can be transferred from one colony to the other. Even a small amount of material from this lysed colony can transfer this lysis effect. If this filtrate is heated, the lysis property is destroyed. He concluded that the lytic agent was virus. D'Herelle rediscovered this phenomenon in 1917. He used the word Bacteriophage for that virus. Bacteriophage means "bacteria eater".

5. Isolation Virus by Stanley

Stanley purified and crystallized the filterable tobacco mosaic virus in 1935. He chemically analyzed  these particles. The particle contained nucleic acid and protein. It suggests that viruses have simple chemical composition.

Classification of viruses

The viruses are classified on the basis of morphology and nucleic acid properties. The genetic material may be DNA or RNA. It may be naked or enveloped. It may be simple or complex. Viruses are classified into following types on basis of their morphology:

➤ Rod shaped. Example, T. M. V (Tobacco Mosaic Virus)

➤ Spherical: Pollovirus. 

➤ Tadpole like: Bacteriophage.

Some viral diseases

 Viruses cause many diseases. Following diseases are common in Pakistan:

1.  Small Pox

Small pox is caused by pox virus. It is a DNA enveloped virus. Small pox is an ancient disease. It occurred in China as an epidemic in 12th century B. C. Small pox was common disease of the world before early twentieth century.

Symptoms

Raised vesicles are formed on the body. These vesicles are fluid filled. Later, these vesicles changes into pustules. These pustules later forms pitted scars. These scars are called pocks.

Control

In 1950, Immunization or vaccination of small pox started. This immunization and other control measures have decreased this disease. It is still present in third word countries like Pakistan. Many people are affected with this disease in these countries.

2.  Herpes Simplex

It is caused by Herpes virus. It is a DNA virus. It is naturally occurring disease of mankind. 

Symptoms

It forms vascular lesions in the epithelial layers of the ectodermal tissues. This disease occurs in the mouth, on the lips and at other skin sites.

3.  Influenza 

It is caused by influenza virus. It is an enveloped RNA virus. Influenza is a wide spread disease. It occurs in epidemic from.

4.  Mumps and measles

Mumps and Measles viruses are belonged to group Paramyxoviruses. They are large and RNA enveloped viruses.

➤ Mumps: It is a contagious disease widely distributed. It is not a fatal disease. About 60% of the adults are immune to it.

➤ Measles: It is a common disease of children and adults. It occurs all

over the world. This disease develops immunity in its victimes.

5.  Polio

Poliomyelitis is caused by polio virus. It is present all over the world. It occurs mostly in childhood. The age of infection varies with social and economic factors.The polio virus is the smallest know virus. It contains RNA in spherical capsid.