I have an epidemic of unknown origin spreading across the earth. The effects are decrease in cognitive ability to around 80+ IQ. So we have a surgeons who don't know how to operate, engineers who don't know math, composers who can't read notes, programmers who can't code, writers who could barely read, judges who don't know the law etc.
On average it takes several years for a complete transformation, with large variance between individuals. At first the effect is very slow, but the ability loss accelerates near the end. Since there is no way to test for the epidemic, beside loss of ability everybody is on a edge. Slight fell of performance and people start to believe that you are infected. Since there is no cure, you could easily lose your job and your college degree becomes useless.
What's minimal percentage of a normal population should I keep in order to keep the civilization running?
I want services like water, sewage, electricity, transportation still running. I want minimum of people who could fix our machines. My aim is to present civilization under severe stress but still barely functioning. Something like a third world country with huge brain drain problem
My aim is something like 10-20% immune, with cutting the rich country services, and retraining remaining humanities, surplus lawyers, bankers, high frequency traders, marketers, non-profit activists, humanities etc.