Virus: 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 ia a helpful virus called bacteriophage which eats bacteria and can not be seen by necked eye. Strangely enough viruses are considered both as living and nonliving objects. They also grow and multiply in living cells and therefore are treated as living organisms. Viruses spread many diseases amongest human beings animals plants and trees. Different viruses spread different diseases. Small-pox, chicken-pox, polio mumps, influenza, hepatitis, mealsles, jaundice, trochoma in eyes etc. are among the main dieases spread by them. Most of the viruses die at high or low temperature. However some of them are immune to the variations in temperature.
Fungus: Fungi are classified flowerless plants having no roots no stems and no leaves and are without chlorophyll – the green colouring matter. Due to the lack of chlorophyll fungi depend for their food either on a living organizm or on the dead organic matter. Fungi feed on dead plants or vegetable matter. There are various kind of fungi mushroom mucedo yeast penicillium mildus rusts blight etc. They are both useful as well as harmful for us. The biggest advantage of fungi is that they destroy and eliminate useless organic matter by setting decay in them. The matter decomposes into oxygen, nitrogen, carbon, phosphorus etc. And they spread in the atmosphere. They also help in preparing bread beer wine and cheese; in making organic acids enzymes vitamins and antibiotics. Penicillin which is usefull in curing many diseases, is prepared from the green-blue fungus called penicillium. How ever on the other hand some of the fungi cause disease in plants and trees and destroy crops of potato apple etc.
Bacteria: Bacteria belong to the lowest category of plants. They are unicellular micro-organisms. They are as small as 25,000th part of an inch. Some of them are even smallerand cannot be seen with the help of an ordinary microscope. They repoduce through the process of cell division ie. one cell divides itself into two. Within twenty minutes the cells of the new bacterium divide into two in forty minutes one bacterium multiplies to four and to sixty four in two hour. In this way in twenty four hours one bacterium multiplies into 4,000,000,000,000 bacteria. If the rate of the reproduction of bacteria remains the same the weight of the bacteria produced within 72 hours will become 33,000 times more than the weight of the earth. But this is impossible because in the struggle for food and water only on percent of them can survive. They may live in soil, water air or in any other organisms. Bacteria are both harmful and useful. They spread many disease in human beings, animals and plants. Typhoid tetanus, T.B., cholera, diphtheria, dysentry, whooping cough etc. are some of the well-known diseases which are spread by bacteria. But on the other hand some bacteria are very useful for mankind. Conversion of milk into curd is done by bacteria. They destroy the dead plants and animals by setting decay in them. Antibiotics and sulpha drugs are used to control bacterial diseases.