Malarial is the adjective that belongs to malaria, the mosquito-borne disease of hot countries. A malarial fever is a fever of that kind; a malarial mosquito is one that can carry the parasite; a malarial region is a place where the disease is common; malarial parasites are the tiny living things in the blood. You do not say someone “has malarial” the way you say they “have malaria”; the -al form hangs on a noun (fever, area, mosquito) or, more rarely, stands as a noun in old medical writing for a person suffering from the disease. It has no extra everyday meaning of its own: it always points back to that illness.