Raju of Class $IX$ was suffering from chickenpox. His friends Priya and Shaurya wanted to visit him. Shaurya's parents did not allow him to go. Priya had already suffered from it a few months before. Priya's parents allowed her to visit him. What could be the reason behind their parents' decision? What is the basis of the principle of immunisation?

(N/A) As Priya had already suffered from chickenpox a few months back,her body has become immune to the chickenpox-causing microbe. So,if that microbe enters her body again,her immune system will recognize and fight it effectively. Hence,her parents allowed her to visit Raju.
On the other hand,as Shaurya has never suffered from chickenpox,his body is not immune to it. Therefore,his parents refused to let him visit Raju to prevent him from getting infected.
The basis of the principle of immunisation: It involves stimulating the host's immune system to develop immunity against a specific disease-causing agent. This is achieved by introducing weakened or dead microbes (vaccines) of that disease into the body. Once the body starts producing antibodies,it can fight against the actual pathogen,and the body becomes immune to that disease.