Everfree
by Nick Sagan

Description:
As Everfree opens, a small group of humans have survived the apocalyptic epidemic called Black Ep, a disease that ravaged the world and left them alone on Earth. Their conflicting ideas about how to propagate the species, however, are a source of terrible strife. Worst of all, they have barely fended off a new strain of the illness that has made a deadly return.

Now, they have bioengineered a new synthetic organ that strengthens the human immune system exponentially. Finally, they have devised the key to their survival. And yet, the organ serves a purpose no human could possibly have imagined, for it will also enable them to communicate with an extraterrestrial species that has appeared on Earth. The disease was a riddle sent by the aliens, and by creating the organ needed to survive it, humanity has answered their call.