Why would a sample of the original virus help to produce a cure?

I watched the first episode of The Last Ship yesteday and it won't spoil anything to let you know that the virologist working to find a cure for the pandemic disease needed to obtain "a sample of the primordial virus" in order to produce a vaccine.



When I first encountered this trope in the very excellent 12 Monkeys, I did wonder what purpose it might serve to have a copy of a virus that wasn't the one you were fighting. However I let it slide. I imagined that there might be some convoluted reason behind this need that was specific to a particular circumstance. It would seem disrespectful to such a masterpiece of cinema to begrudge it a few off camera mysteries. After all, the scientific rationale behind time travel wasn't explained either.



Yet having encountered it again, it's now a thing in its own right. Maybe there is some actual scientific reason why such a sample could be necessary. I'm not a virologist so I don't know nearly enough to discount it for certain. If this is a real thing I'd love to see the explanation. If it is bunkum then I'd like to hear that from someone who knows enough about the field to be sure.