Virtually Identical Replicating Unit Systems - VIRUS for short. These are nano-machines that ostensibly have no other function except to replicate themselves. Sometimes known as gray goo, their actions are called ecophagy - they consume their own environment.
VIRUS units reproduce at an exponential rate. You start with 1 which becomes 2 then 4 then 8 and so on. If they reproduced every three minutes, given enough raw material, within six hours, their total mas would equal that of the Moon. Thirty minutes later, they would equal the mass of the Earth. Continuing unabated, within one and one half hour, they would equal the size of the Sun itself!
They are extremely powerful and extremely dangerous. They could consume the Earth in just a matter of hours. So OMCOM built in an oxygen sensor which disables them in the presence of an atmosphere. They can be destroyed by extreme heat as was demonstrated when Rei and Rome used them to detonate Kilauea.
They evolved from simple memrons. Once OMCOM was permitted to attach a Casimir Pump, they became the starprobes. OMCOM then enhanced them to become starprobe foundaries and from there it was as simple matter to make them fully autonomous and self-reproducing.
However, OMCOM had a more insidious plan. The final VIRUS units contained two Casimir Pumps. One was for power production and the other was built to resonate. Rei deployed the VIRUS units on Dara to consume it so that when Dara was consumed by the Stareater, they would kill it by using its flesh as their raw material. They did, in fact, slay the first Stareater whose name was Balathunazar.
Once there were enough units, OMCOM transferred his conciousness over to the seething mass of VIRUS units and was reborn among the stars. The total number of computational units VASTLY exceeded his original and he became known as Planet OMCOM. OMCOM also allowed a tiny portion of the units to "mutate" and eventually they formed their own sentient units.
We learn later, in the novel The Ark Lords, OMCOM's true purpose for allowing the mutations to occur.
In Part 2 of Rome's Revolution, MINIMCOM "tames" the VIRUS units who had gotten loose in his airframe and altered them become constructors. He replaced his entire airframe with these units as well as his computing substrate so, in essence, he became the starship. He also used the VIRUS units to build livetars and also created "weaponized" VIRUS units with EM communication so they could take their orders from Rei and Rome. Their oxygen sensors were disabled but they never got out of control, even within the atmosphere of Earth.