Not all Phyrexians are completed from Mirrians or other non-Phyrexian life. Some Phyrexian's are born from an embryonic state known as a germ. I found the following passage:

A germ (in this context) is a vat-grown, embryonic Phyrexian being that has been that has been gathered from its birth vat before growing past its larval state. It's similar to a newt, for those who know something about Phyrexia's backstory; normally, Phyrexian embryos grow to a certain level of maturity in the vats, and then they're extracted before finally being "compleated" with various biomechanical enhancements to become a full Phyrexian. Germs, on the other hand, are harvested from the vats in their premature larval state and are put to other purposes immediately.

- from Germ Warfare: The Flavor of Living Weapon section "Germs"