Screw the man-made disasters, just think of how fast diseases would spread. IIRC, dense population centers were one of the contributing factors to the bubonic plague.
Another contributing factor was the lack of modern medicine. Plenty of communicable diseases exist today, and residents of dense neighborhoods are doing fine. If a disease kills too fast for a treatment to be discovered in time to prevent many deaths, it will probably be killing too fast to spread effectively. Swine flu killed ~14,000 people worldwide, so I'd put dying from an outbreak of a deadly disease at the same level of probability as a man-made disaster like terrorism. In other words, not likely enough to be of much concern.