No one realized in time. They couldn't have, as it spread like wildfire. First in eastern Europe, then further.
Millions of dead reported every day.
Governments ordered the bombing of major cities to help contain the infection; it did little.
Eventually, the military took over, governments were discarded and safe zones were built. High walls, crawling with soldiers and with food in rations, the civillians left had to adapt to this new world they lived in as of now.
20 years later.
Present.
The few safe areas left are still controlled by the military which, drunk on power, have no respect for the ordinary people anymore. No one is allowed to leave and no one is allowed in. That's not to say they don't have ways to get in and out. Plenty of groups still live outside, not content with having soldiers dictate when they eat and their curfew.
A group called The Fireflies is the only thing opposing the military, and quite unsuccesfully. With their limited resources, all they can do is try to convince more people to join while stocking up on guns from smugglers.
Children that were born after the infection hit were raised in orphanages, or in schools that trained them to become future soldiers.
Mankind seems hopeless.