Parasitic Diseases. Photo by: Jubal Harshaw. Parasitic diseases are illnesses caused by infestation (infection) with parasites such as protozoa (one-celled animals), worms, or insects. These diseases are widespread in Africa, southern Asia, and Central and South America, especially among children.arasitic Diseases. Photo by: Jubal Harshaw. Parasitic diseases are illnesses caused by infestation (infection) with parasites such as protozoa (one-celled animals), worms, or insects. These diseases are widespread in Africa, southern Asia, and Central and South America, especially among children.