Protozoan pathogens are single-celled organisms that can cause diseases in humans and animals. They belong to the kingdom Protista and are often found in contaminated water or food. Some common protozoan pathogens include Plasmodium, which causes malaria, and Giardia, responsible for giardiasis. These pathogens can enter the body through ingestion or bites from infected insects. Once inside, they can multiply and disrupt normal bodily functions, leading to various health issues. Prevention methods include proper sanitation, safe drinking water, and avoiding contact with infected individuals or animals.