Infested

"INFESTATION describes both a disease and its victims - a metamorphic affliction without cure. Living organisms are consumed and merged into rabid amalgamations. Individuals are violent and animalistic yet the larger whole exhibits signs of coordination, with multiple swarms converging on ships and colonies. Its origins are uncertain but there is historical evidence of a similar outbreak before The Collapse."

- In-game Lore Description

The Infested are what appear to be Corpus and Grineer Humanoids who have been taken over by the "Technocyte" plague. The majority of Infested seem to be former Corpus Crewmen or Grineer Lancers. However they are also speculated to also be partially made of consumed Tenno, hinted at by J3-Golem asking the Tenno, "Why do you destroy us, we are your flesh". This could explain the various powers or abilities some infested have. Most Infested do not have any significant ranged combat ability.

Technocyte Virus
While only mentioned and referenced through in-game text or by Lotus, the Technocyte Virus seems to turn organics such as the Grineer and Corpus into what is known to the Tenno, as the Infested. The name of the virus is the identical to the infection from the first game in the Warframe universe; Dark Sector. The Technocyte Virus, has different properties/symptoms in each game; in spite of both resulting in "zombie" like creatures with no recollection of its previous incarnation.

In Warframe, the Technocyte Virus was created, then utilized by the Orokin to combat the "Sentients". Much of the game's lore is presented in an ambiguous fashion; however it can be inferred that this virus affects species with a common genome. The Grineer and Corpus both being of Earth, as well as the Tenno/Orokin; the virus seems to have no prejudices as to whom it affects. With the J3-Golem stating "...we are your flesh" and itself being a mixture of different types of Infested further asserts the theory of a common genome; as the various types of Infested consist of former Corpus and Grineer. This also infers that the virus in fact has an RNA/DNA base, and in fact is not made of synthetic materials; thus its inability to afftect machines and only organics with a similar genome. An in-game weapon called the Mire references something called the "Great Plague" in its description. The Great Plague may be a reference to the Technocyte Virus itself or a period thereafter. The Mire's resemblance to the Infested suggests the connection between the Great Plague and the Infested themselves.

Walkers
The standard Infested unit. Runners and leapers appear to be bodies of Corpus humanoids, with various alterations and looks. They appear to have a fleshy "hand", perhaps made of Infested tissue, latched to their heads, suggesting the infestation may use this "limb" as a means to control the host during infestation. Their main tactic is to rush the players and overwhelm them with sheer numbers and brute force. It is possible to shoot limbs off humanoid walkers, such as forcing them to crawl by shooting off their legs. Their heads may also explode from headshots without killing them.

Crawlers
Infested walkers without use of its legs. Very slow and relatively harmless, they can only crawl towards their targets. However, they tend to mimic corpses and may be hazardous to unsuspecting Tenno. Crawlers cannot be knocked down and, as with other fallen enemies, are very hard to hit with most melee attacks. Using a finishing move is most effective against one.

Ancients
Ancients are large Infested that are much sturdier than standard Walkers. They have a large amount of health and will charge at players when close enough. Unlike walkers, Ancients can extend their arms to attack, vastly increasing their range. Ancients also come with special abilities, making them a nuisance on their lonesome, and a nightmare in groups. Their weak point is their feet and shins.

They are larger in size than the other Infested units, perhaps suggesting (along with their name) they have grown to their sizes by consuming an abundance of other life forms and may have been the first to be consumed by the plague or created, like J3-Golem.

Phorid
Phorid is the final boss of Eris, inhabiting Naeglar. Resembling an oversized and overbuffed Infested Charger, it bears a possibility of dropping blueprints for the Nyx Warframe and Neurodes resources.

J3-Golem
J3-Golem is the final boss of the planet Jupiter. He can be found on Themisto. He has a possibility of dropping one of the following on death: Volt Helmet, Volt Chassis and Volt Systems blueprints and Neural Sensors. He has also been known to drop 2x mods. He is thought to be the oldest boss, dating back to the Orokin Era.