The antidote can be used to decontaminate the Frozen Leviathan by loading it into the injection machine found near the creature's head and then using the nearby screen. Doing so will cause the pustules on the skin to dissipate, visually showing that the bacterium has been killed. Although the leviathan is frozen (and presumed dead), and therefore has no circulation, injecting a single pustule will cause all the others to disappear as well.
Injecting the antidote is not necessary to complete the main quest and finish the game (in fact, Al-An won't even mention it and will gladly leave the planet before the antidote is injected, despite stating that he was on a mission to eliminate the bacteria). Also, even if Al-An has already left the protagonist's body (and is currently on the other end of the map), his voice will still be heard when the antidote is injected.
The recipe for the Kharaa Antidote will be unlocked when the player picks it up in the Spy Pengling cave. Note that only one dose of the antidote is necessary, so the player only needs to manufacture it if the first dose is misplaced or destroyed.
|Spoiler alert: The following section contains story related material.
|Finding the Cure|
Use the antidote.