The Ampulex Compressa Giganteus originated from the region bordering the Pachacamac River near the Central American town of Santa Cabeza. Upon discovery of the insects, a US government-funded genetic research facility that had been set up near Santa Cabeza was charged with the task of experimenting with the insects' rapid rate of reproduction. Their goal was to harness the wasps' reproductive genes as a means of accelerating cattle reproduction to deal with the exceedingly high demand for meat in the United States.
The research team led by Dr. Russell Barnaby conducted numerous experiments on the wasps and discovered that they were parasitic in nature, requiring host bodies to survive. The researchers used rats and cattle in their tests to determine the wasps' effects on those species. Unfortunately, a wasp escaped the facility and infested a human host, killing it and turning it into a rabid, cannibalistic monster. This led to the parasites spreading throughout Santa Cabeza and turning most of its citizens into the undead.
To stop the zombie epidemic from spreading, the US government ordered the complete sterilisation of Santa Cabeza. The military was sent in under orders to kill off all infected and uninfected citizens, thereby covering up the government's involvement in causing the outbreak.
Since Santa Cabeza, research on Ampulex Compressa Giganteus has been continued by the pharmaceutical company Phenotrans. Following the Willamette Outbreak, Phenotrans has produced and marketed an anti-zombification drug called Zombrex and has made millions by profiting off of zombie victims. They have witheld the development of a cure for the parasites since they can make more money through constant supply and demand. Unknown to the public, Phenotrans have maintained their production of the Zombrex drug by engineering zombie outbreaks in other cities in order to produce more wasp queens to experiment on.
- A fertilized female incubates its eggs within the womb.
- When the parasite discovers a potential host (generally the South American butterfly known as Thysania Agrippina), it injects an egg into the hosts body.
- The virus injected along with the egg effectively prevents the host's immune system from recognizing the contamination, facilitating the larva's growth while keeping the host's physiology blissfully unaware of the danger lurking within.
- The larva excretes a pheromone that stimulates the host's appetite, then absorbs the resultant nutrients to fuel its own growth cycle.
- Once the parasite has grown to the appropriate stage, it devours the host from within, emerging from its cocoon as a full-fledged adult.
ACG queens are responsible for the spread of the zombie virus. They latch on to the back of a human host's neck, so zombies hosting queens can be identified easily as they are always grasping at the back of their necks. If a host zombie is killed, the queen will detach and seek another host. During both the outbreaks in Willamette and Fortune City, Frank West and Chuck Greene were able to capture queens in glass jars and use them for destroying large numbers of zombies. When a queen dies, a violent reaction occurs within nearby zombies that causes the parasite grubs to erupt from their heads. The grubs are normally completely helpless after erupting from their host bodies, but at night they are astonishingly more active. At night, the grubs will glow in the dark and will actually pounce upon nearby potential hosts.