The Bridge — Janine Ellen Young

Somewhere in the cosmos, inquisitive, utterly unhuman aliens reach out, seeking intelligent life, sending greetings and knowledge encrypted within microscopic packets of genetic data. Their information takes on the form of a virus designed to bridge worlds through understanding. But its creators, the alien Kasarans, do not know that their communication might kill...

For when it reaches Earth, the Kasaran virus can't completely adapt to human biology. Instead, it transforms into a plague called the Pandemic that annihilates billions of humans at random.

And when the Pandemic ends, the Earth is changed forever...