I agree with most of what was said here, except that, well.... I don't think it has the potential to actually cause humans to go extinct, or even to simply collapse civilization :-/ Even if a pandemic killed off 75 % of all humans, I have an unprovable feeling civilization would be able to soldier on. This is substantiated by a couple of observations; nearly all human knowledge has multiple backups (pandemics don't kill libraries), so we wouldn't have to reinvent science from scratch. Plus,remaining population would have access to all the material goods of the dead (including canned goods, long lasting food, etc. Which wouldn't be nearly enough to sustain human population for more than a month, but which would give time for people to pick up a book on farming or some such).

On the other hand, it is virtually guaranteed that a pandemic WILL happen (I define pandemic as something that shows up on the news a lot and causes some panic. Kill ratios depend on a case by case basis), given our interconnectedness which is frankly unprecedented in human history (i.e. Microbes, viruses and germs never had airplanes before 1902)