Which one of the following has a long-term effect on the health of an individual?

$(a)$ What is immunisation?
$(b)$ Categorise the following into acute,chronic,infectious,or non-infectious diseases: Typhoid,$TB$,Goitre,Elephantiasis.

You are aware of the $Polio$ $Eradication$ $Programme$ in your city. Children are vaccinated because:

Bacterial diseases are:

What are two approaches to treat any infectious diseases?

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.