Don't forget about that lab that was tinkering around with human-to-human bird flu
, or recent concerns about an airborne ebola virus
. Either is an awesome disaster movie just waiting to happen, but yeah - there's plenty of stuff in the lab and in the wild.
Pandemics and bigger disasters (like Katrina) demonstrate just how fast things can fall apart and people can start freaking out when the going gets rough. During Katrina there were cases of police officers abandoning posts and even becoming looters, as well as hospital staff euthanizing patients (I guess they thought killing them was more humane than letting them die?). Chaos theory suggests that even if you've got a really well designed system, if there are enough variables and strains placed on it that system will not just fail, but probably fail spectacularly.
What is the big one? In the big disaster department, I'm rooting for a really big rock hitting the planet (go big). That way I think we get as many different kinds of doomsday as possible. A big rock would cause massive tsunamis, could also trigger volcanoes around the world to pop, cause a few tectonic plates to shuffle around and serve up earthquakes. Flooding and fires and general overwhelming any kind of emergency services and systems disruption.