In 1993, five billion people caught a virus that had a 21-year incubation period.
During that period, the disease went undiscovered, but caused the intelligence of the infected to rise. Society split into two classes without an understanding of what was causing such a leap in evolution.
Those without the virus, the ten-percent of the human race who were initially immune and everybody born since 1993 were shunned, and forced to live separately. Breakthroughs in technology came at such a rate that a person with normal intelligence was left behind, unable to grasp the new order.
Now, the true spectre of the plague has become clear, as the first of the carriers begin to show symptoms.
The full effects of the disease are yet unknown. It has, so far, been fatal in every case.
The outcasts, without hyper-intelligence, are left to treat the sick.