Someone more knowledgeable in the medical field can correct me, but I don't know any non-manmade virus that is both that contagious and lethal. Not even the bubonic plague.
Most are one or the other. Ebola has a high kill rate but is transmitted by fluids and not airborne. The common cold is highly contagious but hardly deadly.
This is no-screwing-around, global die-off, back to the caves, end game apocalypse stuff.