Name any three diseases transmitted through vectors.

Differentiate between infectious and non-infectious diseases (any three differences).

List three harmful effects of infectious disease.

Choose the wrong statement:

Which one of the following is not a viral disease?

About a hundred years ago,when smallpox persisted,a group of people who had this disease earlier and had survived it had no chance of suffering from it again. So,having the disease once was a means of preventing subsequent attacks of the same disease.
$(a)$ Why does this happen? Explain.
$(b)$ We can 'fool' the immune system. Clarify.
$(c)$ Name this principle of prevention of diseases.