What do you understand by microorganisms? Describe their useful and harmful effects.

Microorganisms are very small and can survive in almost all kinds of environmental conditions. The microorganisms help in decomposition of wastes in sewage treatment plants. Yeasts are unicellular and saprophytic fungi, which are used in fermentation to produce beer, wine and other beverages.
Harmful effects of microorganims are, some of the fungi damage and spoil food products, leather, paper and textiles, whereas others are pathogenic to crops and animals. Protozoans cause serious diseases like dysentery and malaria.