Abominable Infestation
Bio-engineered, constantly evolving species of insect-like flesh creatures. Although resembling insects in some behavioral patterns and sometimes in appearance, the larvae are "asssembled" out of remains of other organic creatures. After growth period has passed, larva grows into an egg that after some time turns into a new caste within the hive. This process(evolution or metamorphosis) isn't exclusive to larvae, as all castes constantly repeat it to grow into more complex creatures seemingly with no limit, although growth and incubation period grows with each such evolution, which could eventually result in hybernation time up to hundreds of years, leaving the egg very vulnerable to attacks. The process can be done voluntarily by each caste, aside from larvae, and as such, they will avoid doing so while in the middle of combat or otherwise in danger.
Origins
The infestation initially began on one of the planets within independent worlds, far away from Sol system. From that point they rapidly spread all across the sector until their territory was simply enormous(~10 times more systems than SolGov currently controls). After that, they chose a different strategy, and launched large amount of "infestation nodes" into all directions to begin new colonies deep in the territory of unsuspecting governments and colonists.
Each infestation node is essentially an asteroid made out of flesh, compressed many times to survive the impact on the planet, which contains up to a hundred of larvae, which venture outside after making contact with habitable planet. Though they are quite vulnerable during first few days, given enough time, they will spread out and consume entire planet and even the system, if left unmanaged.
Nobody really knows of who or what created the very first abominations of their kind. Many speculate that it could've been an experiment gone wrong, while the others suspect it to be a deliberate use of bio-weaponry.
Terran consensus on this matter is that unknown alien civilization launched it to destabilize the human space, saying how the abominations launch their nodes only towards human space. This is, however, not true, as many alien worlds fell just like the human ones. Opinions on SolGov territory vary. Some, taking a more scientific and theoretic approach, consider the possibility of it being a natural evolution process from already existing species on one of the independent worlds. Although government officials did not come up with a unified response, many within it put the blame on independent worlds for "performing dangerous biological experiments that went out of control". Most of the still independent worlds outside of ISC are bordering the sector of infestation, and have to deal with the most vicious of attacks almost every month, and they have gotten quite experienced in the art of exterminating the abominations. Due to having to defend themselves, many dedicated their time to that, instead of debating on their origins.