The Kharaa and the subspecies that make up its hostile presence on various human installations are the product of a previously unknown Bacteria-like growth that rapidly evolves and directly sets in motion the events that lead up to the hostile encounters with humans, and was accordingly termed by TSF scientists as Bacterium. The Bacterium is the basic building block of the Kharaa, and is responsible for the evolution and aggressive behavior of the Kharaa lifeforms. The creatures and structures that the Kharaa have at their disposal were previously not associated with the Bacterium whatsoever, but were separate species that were infested and incorporated into the Kharaa gene pool (including structures such as the Harvester). Once assimilated, these species would be regularly reproduced, albeit re-purposed to better suit their role in the Kharaa, such as carapace and the razor sharp blades that Skulks and Fades evolved.
The terminology for the aliens is still being developed. At the moment, we refer to the aliens, and their whole system of bacteria, lifeforms and growths, as the "Kharaa"; and the pre hive-stage presence as the "bacterium". The first TSF encounter with the aliens occurred aboard the Mongol's Sanjii mining facility. The tattered voice comm logs they extracted from the command network had one word that stood out, used again and again – "Kharaa!" "Kharaa!" Later, it was understood that this means "Watch out!" in Mongolian. By then, it had stuck.
The Kharaa began appearing on ships and bases throughout the Ariadne Arm a decade ago, slaughtering everyone on board. In almost every case the ship then wandered off course, and either destroyed itself by slipping into some gravity well (usually a planet or sun), or was tracked down and destroyed by the owners of the craft. Life support on bases and ships usually fails soon after the Kharaa take over, so even without military response, they don't last very long. However it is still unknown how their appearances on numerous ships and installations can be prevented, and until more conclusive information can be gathered by scientists, humans may never know how or why the Bacterium is capable of setting forth such a destructive onslaught without any early indication or warning.
Initially, the Bacterium only reproduces, slowly forming a mat, visible to the naked eye. All of these bacteria are in contact with one another, sharing data on their environment, and triggering responses to stimuli (like energy sources, or danger). They form a network that links all Kharaa lifeforms and structures, and is responsible for the phenomenon of Hive Sight. It isn't until the presence of larger lifeforms that the next stage of alien growth, the Hive, begins. The hive contains all the information and abilities necessary to set off a cascade of events that rapidly leads to full scale conflict. Like all the other Kharaa structures, TSF scientists speculate that hives are actually creatures that have been evolved into this form over an incredibly long time, and have observed that Kharaa structures in general tend to resemble aquatic forms of life, contrary to the Kharaa lifeforms that clearly originated from mammals and other similar creatures.
The bacterium seems capable of tapping a wide variety of energy sources. Heat and water are diverse enough, but it also finds the rich sludge used for nano-construction highly desirable. Fortunately the same safe guards that prevent marines from automatically accessing these stores thwart the Kharaa too. Unfortunately, the aliens seem quite prepared for this sort of obstacle, building Harvesters atop resource nodes, that draw the sludge up and digest it. These chambers appear to have once been a species, much like the hive. What these resource chambers could have been used for before the Kharaa encountered us is a perplexing mystery. But they convert the sludge into a usable form, just as our resource towers do. As a result, marines and Kharaa end up fighting for the same resources. Defending these resources is vital to the Kharaa, second only to protecting their hives.
To destroy the human presence, the bacterium employs lifeforms of its own. These are the creatures that slaughter the crew of the ship or base, and the same creatures that are waiting for the Frontiersmen when they arrive to take the ship or base back. Kharaa species have natural weaponry and cunning that make them a match for the best equipped and most highly trained forces the TSF can deploy. Hives continue to spawn lifeforms over the course of a conflict, replacing lost aliens. Destroying all hives is the only way to cease the everlasting onslaught.
The Kharaa life cycle begins with the Egg, which is commonly found in large clusters around a Hive. The basic form of the Kharaa, the Skulk, hatches from an egg by biting through the membrane. It has been observed that when the threat to Kharaa territory becomes formidable, the aliens respond by evolving into different and stronger lifeforms as the situation demands. These lifeforms are classified as the Gorge, Lerk, Fade, and Onos, each with their own unique abilities and roles to fulfill.
Kharaa lifeforms are capable of communication through Hive Sight, an instinctive visual mechanism that allows them to identify friend from foe. The Kharaa also possess enhanced senses, more notably Alien Vision which grant perfect visibility in darkness.