Infusion

Infusions are passive upgrades in BioShock Infinite that come in the form of temporally unstable bottles, alternating between three states (red, yellow and blue). When Booker comes across an Infusion, he must choose between upgrading the capacity for extra Shields, Health, or Salts. Once the choice is made the upgrade is permanent, and so cannot be reversed.

Upgrade Choices
Augmenting an attribute with an Infusion increases it by 15% of its base amount, and each attribute can be upgraded up to ten times. There are a total of 24 Infusions scattered throughout Columbia and an additional 5 if the player purchased the BioShock Infinite Season Pass. Added to the shield infusion automatically given to you as part of the story, this means the player can only fully upgrade all of their attributes over the course of the game with the Season Pass. Furthermore, the attributes the player decides to focus on will affect their gameplay to varying degrees, especially on higher level difficulties. The following is a brief guide to choosing which attributes to increase based on the player's preferences:

HealthInfusion.pngh
It is generally not a good idea to increase Health first. Though upgrading Health allows the player to take more damage than by upgrading Shields, unlike the latter, Health does not regenerate on its own (upgrading Health will, however, restore Booker's health bar to full). If the player tends to focus on aggressive combat it is generally advised to upgrade Health after Shields, or even not focus on it at all until the endgame if the player frequently uses Vigors.

Salts
On the tougher difficulty settings, particularly 1999 Mode, upgrading Salts offers the player the greatest benefits, as Vigors are a critical component both in and out of combat. Maxing out Salts first will allow the player to cast two and a half times as many Vigors as they would with the baseline amount, and will also increase the usefulness of each Salt refill (Salt bottles refill a percentage of the player's total Salts). Moreover, upgrading Salts has the added bonus of completely refilling Booker's Salts bar. In combat, a large Salts bar allows Booker to cast more Vigors without having to rely on Elizabeth for a refill, and while exploring it lets the player cast Possession and Shock Jockey on more machines, eventually giving the player an advantage they would not otherwise possess.

Shields
If the player is less reliant on Vigors or tends to take a lot of hits, upgrading Shields first will allow the player to last longer in combat. A larger Shields bar will provide an advantage when fighting aggressively, especially when using close-range weapons. If the player chooses to max out Salts first, it is advised to follow up with Shields, as it will reduce or even eliminate the need to fill up on Health after intense fights. It should also be noted that becuase base Shields (starting value) is lower than base Health, upgrading Shields will give a smaller increase to the bar than upgrading Health early. Upgrading shields is especially effective in combatting boss enemies such as the Handymen and Lady Comstock. During these battles, the sustain granted by a larger shield bar allows the player to keep fighting even when the player depletes the map of vigors and health packs, which often occurs due to the massive health of such boss enemies. For the same reason, vigor upgrades are less effective in these drawn out battles. This makes shields upgrades a good choice especially on 1999 mode because it helps the player the most on the toughest parts of the game where the player may otherwise be stuck in a situation where they repeatedly die due to the lack of resources on the map.

Trivia

 * The infusion bottles use the same selection mechanic as the Nostrum Potluck was planned to before being removed from the final game.