The wirrn did not waste any time.The old geezer tf'd into his natural insectoid form and Nathan and John were strapped into tables,alongside Miss Peabody,their 9th grade music teacher.
They then placed larvae on the 3 humans,which covered the three in a slime which transforms the host into a wirrn drone.
Only when the three had been "turned' did they get an explanation.Back in 1985 a special Dr Who Monster Edition of the Game of Change had been offered.One set had a defect in its magic spell,and when the group of 7 british kids were finished they had been tranformed into wirrn in mind and spirit as well as body.They also discovered that they now possessed the power to transform themselves into other baddies suck as krynoids,fendahleen,daleks,sea devils etc,while retaining their wirrn viewpoint.
This presented both a opportunity and a curse.They discovered that their human minds still existed,but had been subtly altered to serve the best interests of the swarm.In the episode The Space Ark the transformed wirrn retains enough of its humanity to push the destruct button on the space ship carrying it and its fellow wirrn.
The curse was that the newborn wirrn felt an uncontrolleable urge to rebuild their species.Yet the only forms they could take on were those of monsters.They all knew what happens to the monster at the end of the show,it gets bumped off by the good guys.
Thus they slowly cautiously built up their numbers,a runaway here,a missing child there,a eccentric scientist,an accountant.
Slowly the swarm built up its population and its knowledge.John,Nathan and Miss Peabody were just the latest additions to the swarm.