Step 1: Group each disease by its causative microorganism. Step 2: Cholera is caused by a bacterium (Vibrio cholerae). Polio is caused by a virus. Ringworm is caused by a fungus. Step 3: Malaria is caused by the protozoan Plasmodium, which is carried by the female Anopheles mosquito. Step 4: Other protozoan diseases: amoebiasis (Entamoeba) and sleeping sickness. Answer: Malaria.