Now, this could be either a plot hole(unlikely) or a major factor in colony and outside relations.
When Michael first arrives at the colony, he asks Tanya "Just you?", to which she answers yes, whilst also mentioning a few other nurses.
When Michael awakes from his surgery, and listens to Marcus shouting at Tanya, we hear him say "Your not the only other medical authority around here anymore", this point is further supported when it is mentioned that Tanya may not be on duty as the time of Michaels escape from the colony, meaning that a new medical officer may have arrived at the colony during Michael's downtime.
Furthermore, Tanya mentions when it is revealed she isn't a doctor, that she is as good as, saying she has been learning from other doctors during her time at the colony.
This raises two questions, Firstly, what happened to the colony to make them lose the medical experts that Tanya learnt from, did they become runaways? Did they die with Shaun?, the way Tanya talked about it being just her made it seem like it wasn't a recent development, so I lean towards an event we do not know about. Secondly, who is this new medical authority? A maller?
Side Note : Did anybody like the passive aggressive pink cast?