A microorganism is a tiny living organism that can only be seen with a microscope. They include bacteria, viruses, fungi, and protozoa. Microorganisms are found everywhere, from soil and water to the human body, and play essential roles in various ecosystems. Some microorganisms help decompose organic matter, while others are involved in processes like fermentation. Many microorganisms are beneficial, but some can cause diseases in humans, animals, and plants. For example, bacteria can be helpful in digestion, while certain viruses can lead to illnesses. Understanding microorganisms is crucial for fields like medicine, agriculture, and environmental science.