What are protozoan diseases?
Protozoa are single celled life forms and (human) protozoan diseases are typically caused by protozoa that can infect humans parasitically.
An example is "malaria" which is transmitted by mosquitoes that carry a variety of parasitic "plasmodium" protozoa.
When an infected mosquito bites a human to such the humans blood, some of the plasmodium protozoa are transferred into that persons blood stream. They then multiply to cause the malaria symptoms which often result in death.