Project 99 (Gonkillu)

After a series of tests that'd been treated on people illegally underground, one test fails miserably and leads the rest of the world to pay for it.

Killua zoldyck being one of the few who survived the outbreak, now has been alone for 4 years since the world's virus started. Facing the dead and aggressive ever since.

Until he meets another survivor who had made a pit stop on his travel somewhere, Gon freecss. The first he's seen in years.

They'll find a cure.

They will if it's the last thing they do.