The hard part about dealing with a virmitrix, or virmitrice, is that they look like whatever they want. They defy the laws of nature, the laws of biology, and barely adhere to the laws of physics and thermodynamics. Every virmitrix encountered to date has assumed the likeness of the most dominant species, and within a short amount of time, adopted its psychological framework. A virmitrix that adopts a human guise looks, sounds, acts, and even smells human. Thankfully, this emulation is only a few cells deep and revealing one for what they are is relatively easy.
The Virmitrix are plasmid lifeforms, biologically resembling terrestrial molds and slimes. Their point of origin is unknown, but they are not entirely uncommon, which is and should be a frightening reality. Virm biomass is highly adaptable, and capable of multiple forms of cellular metabolism, chemical and toxin resistances, and even resistance to energy and radiation. Virm biomass only has two serious weaknesses, cold and dehydration.
The most prominent weakness of virm biomass is cold resistance. None of the virm metabolic processes are particularly thermogenic so the organism has limited ability to generate its own heat. Exposing Virm to cold will cause them to freeze. This is not fatal, they merely become immobile and dormant. Even shattered a frozen virmitrix will only reduce the creature to a large number of much smaller forms. When these chunks, if thawed, will assume a new form relative to their environment and size. In one instance, a virm fragment thawed, assumed the form of a large rat, chewed its way out of confinement and escaped. The second weakness is dehydration. When exposed to extreme heat and dryness, virm will secrete a dense shell and enter a form of hibernation. Of the two tactics, cryonics are much more effective, as a virm entity can sacrifice its own moisture to advance and act, in contrast to a virm failing in the face of freezing.
The Virmitrix are outsiders, having originated from another Earth, another dimension, where the dominant lifeform developed from slimes and molds and not notochordic animalia. They developed along a slower technological axis, and are completely incapable of reaching parapsychic ability. On the other hand, the virm are mimics by nature, and they consume other creatures whole, and through this process create cellular memory, allowing a virm to replicate any creature they devoured.
When the Virm and Humans crossed paths, the Virm found humanity irresistible. Despite being fixed in shape, humanity possessed unimaginable mental elasticity and capability. They quickly consumed as many humans as possible, and assimilated into human culture, human language, human mentality faster than could have been expected. Likewise, humanity almost immediately noticed the Virm. Despite their cunning mimic skills, the Virm retained many tells, low body temperature, acidic secretions, frequent mirroring behavior, and even the most cursory medical examination could determine the lack of humanity. The shapeshifting ability of the Virm was highly sought after by humanity, for their own nefarious purposes.
In a normal narrative, the Virm would be the villain, appearing and devouring humanity and then ultimately consuming it from the inside and turning it into a new branch of Virm. This isn't the case. Despite their impressive mimicry and assimilation skills, the Virm are not parapsychically active, are easily detected by even basic medical science, and can be conditioned to serve humanity. The most common form of human control is implanting an agonizer or similar pain device. The Virm are mimics, not warrors, not assassins, and are completely unprepared for the absolute ferocity and aggression of humanity.
The Virm could easily flower inside humanity, not overtaking it, not replacing it, but existing within it like a parasite. Unfortunately long periods of Dimensional Fatigue protocols and reflex teams made the incursions of the virm easily noticed and countered.
Humanity, paradoxically, was aware of the Virm, before the Virm were aware of humanity. While the virm gestalt was curious about what was occurring with a handful of their own, humanity had already captured several of them, and were in the process of vivisection and cellular analysis of them. By the time the Virm realized that humanity was the dominant species of this new Earth, humanity had already unraveled the neogenic RNA of the Virm, and had figured out their metamorphic ability, and were in the process of domesticating captuted Virm and creating anti-virm vaccines.