Which one of the following causes kala-azar?

Educating parents would help a lot in reducing the incidences of diseases in children. Justify the statement with five reasons.

What are two approaches to treat any infectious diseases?

Distinguish between acute and chronic diseases.

Which disease is not transmitted by mosquitoes?

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.