Let's say the Spanish Flu kills 90% of the human population in 1918-1919, but, instead of disproportionately killing off healthy young people, the lethality of the flu has a direct correlation to how old you are. In this sense, most educated people are toast, but documents of technological advances still exist.
How long does it take, by your estimate, for people to reach WWII level technology?
And, as a bonus, do you think human civilization would collapse and if so how long would such a collapse last before new nations begin to arise from the rubble?