(a) What is the b

(a) Immune system develops a memory for a particular infection when it encounters it for the first time. So, when the next time that particular microbe enters the body, the immune system responds against it with even greater vigour.

(b) Because they are already exposed to the Hepatitis A virus through contaminated water.

(c) (i) Bacterial diseases - cholera and tuberculosis. (ii) Viral diseases - common cold and influenza. (iii) Fungal diseases - Food poisoning and skin infections.