Malaria is a disease that effects hundreds of millions of people worldwide, every year. It is estimated that malaria kills three million people a year, half of which are young children. Malaria is caused by several protozoa in the genus Plasmodium. This particular protozoa carries out life cycles in both the digestive systems of mosquitos, and in human blood cells. The unicellular protozoans enter the bloodstream of humans during a mosquito bite. They find an uninfected red blood cell, and use a special combination of organelles at the end of their cell to force the cell to draw the parasite in. This special group of organelles, known as the apical complex, allows the parasitic protozoa to hide within blood cells. This makes Plasmodium and other parasitic Apicomplexans very hard to kill, as the blood cells would have to be destroyed as well.

An interesting phenomena in the ocean is also caused by another phylum of protozoa. The Dinoflagellates are a group of protozoa that exist mainly as free-swimming unicellular organisms, while other exist symbiotically within certain invertebrates. Groups of dinoflagellates that are free swimming can undergo drastic periods of reproduction, known as blooms. If a bloom produces enough dinoflagellates, the ocean water will become pink or even red because of their density. Unfortunately for any organism that passes through the bloom, dinoflagellates produce a toxin. When congregated in enormous densities, this neurotoxin can paralyze and kill almost any organism in the bloom. Fish, crustaceans, and mollusks that eat the bloom may store the toxins in their tissues and become poisonous to people who eat them. Paralytic Shellfish Poisoning is the caused by dinoflagellate toxins in high concentration. Simple protozoa, in high numbers, can cause muscle paralysis and respiratory failure in humans, possibly leading to death.