Not to be confused with the first game's version, Infusion.
The Infusion is a returning unlockable Uncommon item in Risk of Rain 2. The user's max life is increased by 1 whenever they kill an enemy, up to 100hp per stack. Having multiple stacks will increase the amount of max life gained per kill, as well.
The Challenge "Slaughter" unlocks this item.
The user has to deliver the killing blow themselves or have the enemy die to an effect they made for this item to work. Enemies killed by Auto-Turrets increase the Turrets max life instead, and kills made by drones or ghosts won't give the bonus.
Visually this item is shown as a series of three red vials on the characters right thigh.