EDIT: Everybody's infected, time to screw the world over.
What is infective but not noticeable?
Fever is the only thing I can think of, you'll have to use sneezing though if you want it to happen faster.
Needs zombies.
Then call your disease the Rage VirusBonus points for the one to point out reference
loving AMERICA HAD 10 PEOPLE LEFT WHEN THE VACCINE CAME OUT. 10. ONLY 10. WHAT THE forget. THEY LIVED, THE REST DIED.
At least they can face the horrors of millions of dead bodies. And know that everyone else in the world is dead.