Virus:

Virus is a very tiny, infections and highly pathogenic organisms, which can multiply only within the living host. They don't have cellular structure. They contain hereditary material in the form of DNA or RNA enclosed in a protein coat called capsid. A mature virus is called a viron.

They exhibit properties of both living and non-living things, as Virus has no metabolic activity of its own.

They are smallest living organism with the size of about 20nm to 300 nm. Based on the host they attack, viruses are of 3 types.

·    Animal Virus

·    Plant Virus

·         Bacteriophages

They are also classified into RNA virus and DNA virus on the basis of nucleic acid present in them.

Q. Why is a virus considered as the boarder line between living and non living things?

Virus is a living thing only inside a cell and nonliving thing outside a cell so it is considered as a chain between living and non living things.

Q. Why are viruses called obligatory parasite?

                   Viruses are called obligatory parasite because they are fully dependent on the other organisms.

Characteristics/ Nature of Virus

·         They don't show respiration & metabolism

·         They can reproduce only inside living cell.

·         They don't show cellular structure.

·         They are like molecules.

·         They show host specificity i.e. particular virus can attack only a particular host

Transmission of Virus:

Virus transmits by means of air, water and food particles. When an infected person sneezes, virus from the water droplets enter the near by healthy person. Viruses also transmit by blood and body fluids. Some common viral diseases, organs affected medium & control measures are shown in the table below.

Name of the disease	Name of Virus	Organs effected	Medium	Control Measures
Common cold	Rhino virus	Upper respiratory tract	Saliva	Control of infection
Mumps	Parmyxo virus	Respiratory tract	Saliva	Control of infection
Measles	Parmyxo virus	Mouth, Skin, Instestine and Respiratory tract	Saliva	Control of infection
Polio	Polio virus	Pharynx, Intestine, Blood and Brain	Feaces, Saliva	Polio drops
Rabies	Rabies virus	Spinal cord, brain	Saliva	Anti rabies vaccine
AIDS	Retro virus	Skin, Blood and Brain	Unsafe sex, Blood	Avoid sex with multiple partners, Use condoms

Q. Culture of virus is impossible, why?

            Culture of virus is impossible because it is very minute and it is alive inside host only.

Exercise for Practice

1.      Identify the two diseases caused by virus regarding the parts affected.      

a.       

                                                              i.      High fever

                                                            ii.      Respiratory tract becomes affected.

                                                          iii.      Eyes become sensitive to sunlight

                                                          iv.      Skin rashes

b.       

                                                              i.      Respiratory system,

                                                            ii.      fever is intermittent

                                                          iii.      parotid gland

                                                          iv.      Testes.

2.      Write two characters of virus.                              

3.      Why virus is called obligate parasite?

5.       

a.      Why are virus known as obligatory parasites?

b.      AIDS virus doesn't kill a human being but it helps other diseases. Justify this statement.

6.      What is the reason for transmitting HIV virus causing cold through blood and air media    respectively?  

7.      What causes polio?                                                    

8.      Which virus causes common cold? What happens if a person suffering from AIDS catches common cold

9.      Are viruses living or non-living objects? Explain.                             

10.  Give full form of AIDS and TMV.   

11.  Write the full form of AIDS. State four main methods by which the virus that causes AIDS gets transmitted from one person to another

12.  How does individual suffer from AIDS? Give a way of transmission of this disease. What symptoms is seen in the victim?

13.  Give Characteristics of Virus

14.  What is virus  

15.  Why do polio victims become paralyzed?