[quote=@The Grey Warden]
So, do they bite the citizens and turn them into slaves
[/quote]

Yes, once bitten, or even if one gets too close to a zombie, their life force starts to be drained. A bite almost immediately means you're dead.

[quote=@The Grey Warden]
that can transform the parasite?
[/quote]

I don't understand this part of the question. However once a person is drained of their life force, they essentially become a black dried out mummified husk of who they used to be and act as part of the parasite's hive mind.