A protozoan parasite is a single-celled organism that can live and reproduce inside a host, often causing disease. These parasites belong to the kingdom Protista and can be found in various environments, including soil and water. Common examples include Plasmodium, which causes malaria, and Giardia, responsible for gastrointestinal infections. Protozoan parasites can enter the human body through contaminated food, water, or insect bites. Once inside, they can multiply and disrupt normal bodily functions, leading to symptoms like fever, diarrhea, and fatigue. Understanding these organisms is crucial for preventing and treating the diseases they cause.