Which one of the following is not important for individual health?

Health is not merely the absence of disease. How can we define health? Classify diseases on the basis of:
$(i)$ Duration of the diseases:
$(ii)$ Cause of the diseases. Give one example of each type.

During a disease:

Students of class $IX$ had to present a seminar on 'Dengue fever'. They wanted to introduce the disease by starting with:
$(a)$ Immediate causes of the diseases
$(b)$ Definition of infectious diseases
$(c)$ Pathogen causing dengue fever
$(d)$ Vector responsible for dengue microbe.
Can you help them to explain the above points? Give details.

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?

What are two approaches to treat any infectious diseases?