Protozoa are responsible for numerous diseases, including: - Malaria: Caused by various species of Plasmodium. - Amebiasis: Caused by Entamoeba histolytica. - Giardiasis: Caused by Giardia lamblia. - Leishmaniasis: Caused by Leishmania species. - Toxoplasmosis: Caused by Toxoplasma gondii.