(a) The virus of Hepatitis A spread through contaminated water and there are many children below the age of 5 who get infected to this disease through the polluted water. The infection of this disease is mild during early stages of life i.e. childhood and by the time they reach adulthood their bodies develop large number of antibodies for this disease as a result majority of children in many parts of India are already immune to Hepatitis A. This is an example of active immunity.

(b) Acute diseases are the ones which are severe, have a sudden onset but last for a short period of time, example, a broken bone. While chronic diseases are the ones which are long lasting and kill the person from inside, example, osteoporosis. Osteoporosis is definitely more harmful than a broken bone. This is why, it is said that chronic diseases cause more harm to the body than acute diseases.

(c) The diseases which can be transmitted from one individual to another are known as infectious diseases or communicable diseases. These diseases can be of different type depending upon the infecting microorganism like bacteria, fungi or virus. Example of infectious diseases are cholera, common cold, typhoid, HIV/AIDS, malaria, tuberculosis, etc.