500,000,000
Invent a 'super vaccine' that protects you from all viruses. Invoke some kind of new nano tech to make it convincing. Hail it as the greatest medical advancement in history.
The sheep are all scrambling for it. They 'apply' to be one of the 500,000,000 limited recipients. Countries conduct contests and lottos to decide who will be 'awarded' the chance to receive the miracle vax. A life of no-hassle travel and work is guaranteed, the super-vax is viewed as the ticket to the future.
But it turns out something in the vax creates a gain-of-function mutation in the recipients. While they enjoy immunity, they inadvertently infect everyone else with a dangerous and deadly strain of the virus. It has a 90% mortality rate with no known cure. 7 Billion people die.