This is how it happens now.
A little-known virus begins infecting people in one small corner of the world. As it spreads, the rate of infection speeds up. It gathers steam, transcending borders and spanning oceans before finally triggering a global response. But the genie is already out of the bottle.
This time, the virus in the news is Zika. As of Aug. 3, Zika had spread to 65 countries.