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. Man is only an intermediate host and not the definitive host.
• B. Vaccines can be developed only against bacteria.
• C. Man does not develop immunity to malaria during natural infection.
• D. Malaria is caused by several species of Plasmodium.