During a visit to a hospital, a child receives the oral polio vaccine. He then returns to his distant village. Sometime later a polio outbreak occurs in the village, but the child and his siblings, who had not had the vaccine, are spared. What is the explanation for this event?

Because the child was vaccinated against this disease, the germs are not able to travel from one person to the other, and the family in this case, is less likely to get the disease. Even his siblings, that didn't get the vaccine, will get some protection from getting sick. This is known as community immunity.