A microorganism is a tiny living organism, typically too small to be seen with the naked eye. This category includes bacteria, archaea, fungi, protozoa, and some algae. Microorganisms play vital roles in various ecosystems, contributing to processes such as decomposition, nutrient cycling, and disease dynamics. They can be beneficial (e.g., in digestion) or harmful (e.g., pathogens), making them essential for both health and environmental sustainability.