The correct options are
A If a disease has high mortality rate.
B If the disease is not prevelant in the present but has a high chance of appearing again in the future.
C When there is a risk of contracting a particular highly pathogenic disease.
Vaaccines are biological preparations which help in acquiring resistance against an infectious disease. It is necessary to get vaccinated for diseases which can be fatal, are highly infectious and risks of contracting the disease is high. Though the disease causing pathogen may be dormant in the present, it is better to be vaccinated if there is a chance of relapse of the disease in near future. Certain vaccines can also help in the cases of recurrent infections.