Microorganisms are tiny living organisms that can only be seen under a microscope. They include bacteria, viruses, fungi, and protozoa. Bacteria are single-celled organisms that can be found in various environments, including soil and water. Some bacteria are beneficial, helping with digestion, while others can cause diseases. Viruses are even smaller than bacteria and require a host cell to reproduce. They can infect plants, animals, and humans, leading to illnesses like the flu. Fungi, such as mushrooms and yeasts, are also microorganisms that play essential roles in decomposition and food production. Protozoa are single-celled organisms that often live in water and can be free-living or parasitic.