What is the purpose of vaccines?
to help your immune system prepare for diseases
to kill existing diseases in your body
to speed up your healing from diseases
to weaken diseases that affect your body

to help your immune system prepare for diseases

Explanation:

To help you immune system prepare for diseases. Vaccines are basically weakened versions of the original sickness that your body fights off and develops antibodies to defeat the disease in the future.

the present continuous is a verb tense which is mainly used to indicates that an action is in progress at the moment of speaking.  

it is formed using: person + verb to be (am/is/are) +verb + - ing form  

example: i am studying at this moment .

in addition, the present continuous is also used in order to express things that will or will not happen in the near future.

example: i am having dinner with some friends tonight.

the speaker sometimes   uses it   to express that an action is repeated and it causes irritation on the part of the speaker

example: my sister is constantly complaining about everything.

he was looking for nothing because this question had no complete sentence.