It spreads via saliva and survives on surfaces, and the "symptomatic" period starts like a fever. It's not as if people go from being healthy to bleeding from every orifice in 24 hours. It's a hard disease to stop because it initially looks like a thousand other common illnesses.

Again, we're telling ourselves comforting bedtime stories when we pretend that there's something innate to the virus that makes us safe. We've just never had an outbreak in a city before, so we've had the advantage of geographical isolation.

Take a few incipient cases and stick them in a tenement in New York or Paris, and the epidemiology can become incredibly complicated before we even know we have a problem.