(a) Explain the following terms:
(i) disease
(ii) symptoms of diseases.

(b)(i) List two physical and two chemical barriers that prevent pathogens from penetrating the body of an organism.
(ii) Explain how vaccination protects the body from contracting infectious diseases.

(c) Distinguish between an antibody and a antigen.

(d) Name the causative agents of:
(i) Malaria
(ii) Cholera
(iii) AIDS.