Microorganisms play both useful and harmful roles. Useful microorganisms aid digestion by producing vitamins, help decompose dead materials through enzymes, and are used to produce medicines, health supplements, compost, and in various industries like bread and alcohol production. However, harmful microorganisms can cause diseases that are treated through antibiotics, vaccines containing weakened forms of pathogens, or other medical interventions. Microorganisms are classified and their growth is affected by various environmental factors.