It is just a guess since I haven't seen any raws or anything, but I suppose the doctor is descendent of the boy that escaped of the island and I think he is resistant to the disease because of that and that is why he is so important for the researchers. But I could be completely wrong. ╮(￣▽￣)╭
From what I see first chapter is one-shot. That's why it's chapter 0.
Most authors do one shots before they start serialisation. If history is liked by editorial team and readers, they gets serialisation. But very often serialisation is really different story than one shot because author has to consider remarks of his editor.
WAIT!! WHAT ABOUT THE GIRL Σ( ￣□￣||)!! DID SHE ESCAPE??
WAS THE DOC FORM THE FIRST CHAPTER THE BOY FROM THE SECOND ONE!!??
I GOT SOOOOOOOOOO MANY QUESTIONS!! Σ(っ°Д °;)っ