(a) Yes.
Because functioning of immune system will not be good if proper and sufficient food and nourishment is not available.
(b) Yes.
(i) For air borne microbes, we can prevent exposure by providing living condition that are not overcrowded.
(ii) For water borne microbes, we can prevent exposure by providing safe drinking water.
(iii) For vector borne diseases, we can prevent exposure by providing clean environment.