Infusions (they're also called essences) are used to improve the main character's primary statistics: health, shield and salts. The first infusion you'll receive is called shield. It won't be an upgrade, because you will receive a basic shield itself (you'll get it from Rosalind Lutece after reaching The Blue Ribbon Restaurant in chapter 4).
The shield mentioned above works like an electromagnetic barrier. It absorbs some of the damage and it eventually breaks. Once this occurs the shield will need a few moments of peace to regenerate itself. The time and speed of shield's regeneration are based on the difficulty level:
- Easy difficulty: 3 seconds [regeneration delay], 1 second [regeneration speed]
- Normal difficulty: 4 seconds [regeneration delay], 2 seconds [regeneration speed]
- Hard difficulty: 5 seconds [regeneration delay], 3 seconds [regeneration speed]
- 1999 Mode difficulty: 6 seconds [regeneration delay], 4 seconds [regeneration speed]
Each infusion should be considered as "three bonuses in once". Once you've taken the essence you will be allowed to improve one of the statistics (up to ten points). Sadly you'll find only 24 infusions throughout the course of the entire game and this means you won't be allowed to fully upgrade all of your statistics. You must plan your character development right from the start.
If you have a collector's edition or if you've pre-ordered BioShock: Infinite, then you will be given an opportunity to receive five additional infusions at the beginning of the game. The essences (much like gear) will be waiting for you in the Blue Ribbon Restaurant (chapter 4).