Hint Micro-organisms are also known as microbes which are very small in size, they include bacteria, protists, fungi, viruses, viroids, and prions. Bacteria and fungi are both harmful and useful for living organisms while others produce only harmful effects.
The harmful effect of microorganisms like bacteria includes spoilage of food, food poisoning, deterioration of domestic articles, denitrification of soils and diseases in plants and animals. Spoilage of food caused by saprophytic bacteria which cause rotting of meat, fruits, vegetables, the souring of milk and spoilage of jams, jellies, and pickles. Food poisoning caused by Staphylococcus aureus, Clostridium botulinum, and Salmonellosis. Cotton fabrics, leather, and wooden articles deteriorate by bacteria Spirochaete and Cytophaga. Denitrifying bacteria like Thiobacillus denitrificans and Micrococcus denitrificans cause waterlogging in soils and convert nitrates of soil into gaseous nitrogen. Bacteria are parasitic in the case of both plants and animals, they draw nourishment from the host and lead to diseases in human-like pneumonia, cholera, tuberculosis, leprosy, tetanus, diphtheria, syphilis, gonorrhea, etc. Plant disease includes rotting and rust. Protists mostly cause human diseases like malaria, amoebic dysentery, sleeping sickness, kala-azar, dum-dum fever, giardiasis, and leucorrhoea. Yeast causes green or blue mold on several food articles. It also causes ergot of rye, powdery mildew of cereals, bakanae disease of rice, the early blight of tomato and potato, wilting of crops, and smut on cereals. Viruses cause disease in plant-like tobacco mosaic, potato mosaic, pumpkin mosaic, wheat mosaic, tomato bunchy top, banana bunchy top, rice tungro, etc. The animal disease includes foot and mouth, encephalitis, rinderpest, whereas disease in human is poliomyelitis, rabies, mumps, measles, chickenpox, smallpox, dengue, common cold, flu, yellow fever, herpes, hepatitis, AIDS, etc.
Note: In animals, 90% of diseases caused by bacteria and in plants, 40% of diseases are caused by bacteria. The virus causes very less number of diseases but it is more harmful than bacteria because it directly attacks the DNA of the cell.