From what we've seen under scopes, the extracted human hairs, in the presence of "morg", seem to be (abnormally) opened up at one end (follicle). This is where infection can happen easiest, I think, since the hair cuticle is softest there. They can probably infect a hair in situ (i.e. still in your head), and again, it's probably easiest at the root/follicle.
If the pathogens already have encountered a hair before (in their evolutionary life) then assuming they have the technology, they can likely produce a hair through their own RNA replication processes; this would explain the "growth" of a hair from a pore where there was not a hair growing before.