Infusion

Infusions are recreational narcotics used by the Unggoy, who add them to their portable methane tanks. During the mid-2400's, as a revenge for Kig-yar nest relocations caused by the rapidly increasing Unggoy numbers, some radical Kig-yar poisoned Unggoy infusion supplies, making several thousand affected Unggoy males sterile. The Covenant leadership's lack of response to this event, which became known as the "Infusion Incident", eventually caused the Unggoy to revolt, beginning the Unggoy Rebellion.

Trivia

 * The Final Grunt in Halo 3 says "When I'm on the gas, I don't know what I'm doing half the time!". The "gas" may be referring to a type of infusion.