Inhabitants of the Andromeda Galaxy, the Wirrn were a parasitic race of giant alien insects resembling Earth wasps, that swarmed across sectors of space, laying their eggs in living hosts and furthering their colonies. when the eggs hatch the Wirrn pupae can then infect other life-forms converting them into more Wirrn but retaining the knowledge and intellegence of the the original host body. The Wirrn had a grudge against humanity claiming that humans had invaded their colonies and fought them for over a thousand years.
When a Wirrn queen came across the cryogenically frozen survivors of humanity on the Terra Nova (Nerva) space ark she laid her eggs inside one of the sleepers. The Wirrn then began to use the station's solar stacks to breed in. Eventually, the Wirrn took over the Ark's leader, Noah, who began to gradually transform into a new Wirrn Queen. The Wirrn originally intended to use the Ark as their a breeding ground, but when when the Doctor prevented their access to the sleepers, they attempted to use the escape shuttle to settle elsewhere. But Noah's mind rebelled and, with his last shred of humanity, he activated the self destruct mechanism blowing up the shuttle and destroying the Wirrn swarm.