[natrius]

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.