Parasitic protozoa are single-celled organisms that live in or on a host organism, deriving nutrients at the host's expense. They can cause various diseases in humans and animals, such as malaria, which is caused by the genus Plasmodium, and amoebic dysentery, caused by Entamoeba histolytica. These protozoa often have complex life cycles that involve multiple hosts. These organisms are typically transmitted through contaminated water, food, or insect bites. Once inside a host, they can reproduce and spread, leading to health issues. Understanding parasitic protozoa is crucial for developing treatments and preventive measures against the diseases they cause.