Volatile infested were made from the infested research team on Jarban Minor after the zerg overran them. They appear to be similar to normal infested colonists, but with a large explosive pustule on their chest, which explodes when it gets in range of a target.
When Agent Nova Terra arrived on Jarban Minor in search of terrazine, she was forced to fight off waves of these creatures, which swarmed the extraction devices as she tried to harvest from them.
Volatile infested appear in Co-op Missions as a possible unit spawned by Alexei Stukov. Stukov's tier 2 mastery gives him a 0.5% chance per mastery point (max 15% chance) to spawn a volatile infested instead of a regular infested civilian. In addition, volatile infested are fired by infested siege tanks, and are controllable after they land next to their targets.
Volatile infested appear in the Nova Covert Ops mission Night Terrors. They function as slower moving banelings, but have higher health. When harvesting from an extractor, waves of them will often appear from nearby caves, particularly those from the southeastern most extractor. Area-of-effect such as siege tanks or Nova's grenades can do well against clustered volatile infested. In addition, HERCs are immune to their acidic explosions.