It's happened before with the Spanish Flu and the Black Plague, which wiped out almost a third of the world's population at the time. However, in order to truly be a threat to society as we know it, an illness would have to be transferrable to other species. Otherwise, the illness gets less potent the more people it kills, because the population density thins. Now, naturally, we've encountered several diseases that do just that, however none of them have spread fast enough to make us seriously question whether the human race will survive.
How likely is it? Not likely, but, somewhere, scientists are hard at work every day making new and more terrifying viruses (we assume). Even with all our science, though, for all we know, the bug that leads to human extinction is hidden in some creature deep in an unexplored jungle. 4 of 10 points
How cool is the post-apocalyptic scenario? Not as cool as a zombie apocalypse, that's for sure. If a zombie apocalypse is like a paintball field, then a pandemic apocalypse is like wandering through the sick ward at the children's hospital, watching everyone you've ever loved choke on their own bodily fluids. 5 of 11 depressing points
How likely is it that humans will survive? As mentioned before, it would take an extremely adaptable virus to survive its own devastation. Humans would almost definitely survive, although in what shape we'd be is up for debate. 9 of 10 points
How awe-inspiring is it? A bunch of people coughing themselves to death? We expect more from our mass extinction. 1 of 11 points
Overall Score: 19 of 42