More than five months ago, Mercy had perfected her Nano biology that allowed her to stay young.
She had tested it on other humans to varying degrees of success and the nanobots where there sold to the public.
They spread like wild fire everyone wanted some unknown to them the nanobots caused varying degrees of side effects like, Paranoia, Sickness, the need to eat more, it was also proved to cause adhd and other mental disorders to the user.
Mercy of course was greatly hated for this in most famous court case ever blamed this on omnic hacking into the nanobots and changing them to do these effects.
There was a massive investigation of Angele Ziegler and she was proved innocent to the nanobot drama. But by then the nanobots were called back before they hit the other continents.
Someone had then infected the nanobots with an unknown virus that caused violent behaviour and frequent outburst the whole of North and South America were going into chaos foreign countries refused to help them to stop the virus further spreading.
Then people started to turn into zombies and all chaos went loose. The authorities couldn't afford to stop these and the zombies spread quickly.
Mercy was also was found dead with multiple injuries suggesting an assault on her. Preventing a cure from being made.
Overwatch was sent to contain the outbreak they even teamed up with talon to contain the outbreak but then they were stranded by the un trying to get rid of both rival terrorist groups.
They now must survive in the zombie apocalypse
A/N
This is just an idea i have review for the characters you want to see next.