Invoked: A terrorist group creates the virus and unleashes it on the countries they hate most.
Defied: Doctors work around the clock and find a cure before it's too late.
After the virus hits, humanity gets hit hard. All of the millions of essential personnel who disappeared will never return, meaning their skills will never be recreated. The basics of a society's infrastructure - like agriculture, farming, and construction - go unfulfilled, meaning society itself collapses.
90% of the population is still a lot of people, and the protagonists would probably have had someone close to them die from this, which can leave some psychological scars, especially if someone was immune and had to see their family and friends die before their very eyes.
Reconstructed: The survivors have to learn to adapt - which is what happened in the past anyway.