Decimation of Mankind

People have invented a flu, this flu was never test. First the flu was successful, no problems. More and more people were injected, till one little change was made, one thing that would change humanity. Everyone was injected that was older than 15.
More and more people where dying after two years of taking the injection. This was worse than the Spanish flu, most doctors and scientist where developing a cure and vaccine. Then they were finished but there was never time for it to be put to use before every adult died. Now humanity depend on the remaining people. It was also thought to be a disease that affected people after the ages of 15, but no one knows if it will spread.
The remaining people will have to find the vaccine and cure in order to survive over the ages of 15. Now humanity's hands are on the remaining humans.