Why only 100M? The Plague wiped out half the world's population, and travel was far less speedy than it is now. Granted the carriers were ubiquitous (fleas). But as I recall villages and towns would close themselves off, and outsiders risked being killed if they approached. So the slow travel was further restricted, and it still killed half our species.

Today I would think it could spread like wildfire before anyone realized there was a pandemic on the move. And while response would be swifter, I can still imagine entire cities being quarantined with people dropping like flies. Up to 130M people died of the Spanish flu, and travel was not very rapid (and with a war on I suspect it was quite restricted also).

With our higher population, and population density, you don't think it would be upwards of 1B?

1poorguy