Who discovered the 'vaccine' for the first time? Name two diseases which can be prevented by using vaccines.

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

What is a disease? How many types of diseases have you studied? Give examples.

State whether the following statement is true or false:
Chronic diseases have very drastic long-term effects on people's health as compared to acute diseases.

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.

Why is the immune system essential for our health?