Most Likely

[King of the Wastes]

I think that this is the most likely to happen apoc scenario, even if other ones happen I think a viral outbreak will still play a role.

[blightedbaz]

H5N1, read up, once it makes the leap from human to human we are screwed

[imperator03]

Storm in a teacup. If you look at the great pandemics in history, they have all followed massive movements of people, malnutrition and war. The last great pandemic we had was during WW I and that was a strain of the Kansas flu that migrated to France and mutated in the hellhole of trench warfare to become the killer it became. What surprised me is that we didn't have the same thing happen after WW II although the Hong Kong flu could have gone through the same metamorphosis during the Vietnam War. Should my theory prove correct, the limited role of that conflict could have lessened the virulence of the HK flu so that fewer people died than from the "Spanish" flu.

[King of the Wastes]

I don't make response's so well, but imp that does make some sense. Though disease can be spread through trade routes as well I think, like the silk road.