Which one of the following causes kala-azar?

The role of an active immune system is:

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.

Define an infectious disease. Give any one example of it.

$(a)$ What are the two ways by which an infectious disease can be treated?
$(b)$ Explain the mode of action of antibiotics.

We should not allow mosquitoes to breed in our surroundings because they