Question: Widespread resistance of malarial parasite to drugs like chloroquine has prompted attempts to develop a malarial vaccine to combat malaria. Why is it difficult to develop an effective malaria vaccine? A. Malaria is caused by several species of Plasmodium. B. Man does not develop immunity to malaria during natural infection. C. Vaccines can be developed only against bacteria. D. Man is only an intermediate host and not the definitive host.