Widespread resistance of the malarial parasites 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