Viruses are micro organisms which cause various diseases in all types of living bodies, so much so that even bacteria are affected by viruses. There is a helpful virus called bacteriophage which eats bacteria and can not be seen by necked eye. One can only see them with the help of an electron or ultra microscope. Strangely enough, viruses are considered both as living and non-living objects. They grow and multiply in living cells and therefore are treated as living organisms. Since they do not grow outside living cells. They are classified as non-living objects also. Consequently, they are treated on the border line of living and non-living matter.
Viruses were first discovered by Mayer in 1888. He found them on tobacco leaves. There are many kinds of viruses. They are of varying shapes and sizes. Some are long as rods, some are hexagonal. The smallest viruses are of the size of a millionth part of an inch. They are found in the cells of plants, animals and human beings. They are born in these cells, multiply there and spread diseases.
Viruses spread many diseases amongst human beings, animals, plants and trees. Different viruses 
spread different diseases. Small-pox, chicken-pox, polio, mumps, influenza, hepatitis, measles, jaundice, trochoma in eyes, etc. are among the main diseases spread by them. Some of them cause brain diseases such as paralysis and rabies. Some kinds of cancer too are caused by viruses. Many diseases in the plants of tomatoes, bananas, sugar-cane, lemon and cotton are also caused by them. Thus we see that they are among the greatest enemies of living beings.
Most of the viruses die at high or low temperature. However, some of them are immune to the variations in temperature. Chlorine and hydrochloric acid are used to kill them. There is very little doctors can do to treat viral diseases directly. Our body fights viruses in two ways – by producing antibodies and interferon. Antibodies clump viruses and destroy them. Interferon helps keep viruses from spreading cell to cell. Many vaccines have been developed to give artificial immunity against viral diseases.