Microorganisms are tiny living things, some helpful and some harmful. Useful ones like Lactobacillus make curd, yeast helps in baking, and Penicillium makes antibiotics. Harmful ones, such as E. coli, Salmonella, and certain fungi, cause diseases like diarrhea, typhoid, ringworm, and athlete’s foot. Microscopic algae produce oxygen and support aquatic life, while protozoa like amoeba and plasmodium can cause illnesses such as malaria. They play important roles in food, medicine, and the environment but can also spread diseases.