Pneumococcal conjugate vaccine is given to prevent the spread of diseases caused by pneumococcal bacteria. These bacterias are responsible for illnesses like pneumonia. You can read about the Zoonotic Diseases – Definition, Types, and how they spread in the given link.

Further readings:

1. Science & Technology Notes For UPSC – General Science Notes 
2. Science & Technology Questions in UPSC Mains GS-3