The Beacon (Earth Haven: Book 2) - Earth Haven, #2

A post-viral planet. Annihilation approaches. Tom Evans and Ceri Lewis occupy a silent new world. Little more than a million people survived the Cleansing. They are spread out, weak, compelled by a force beyond their comprehension to remain where they are. Humankind teeters on the brink, its fate in the hands of the few who can resist the power that seeks to control them. But resistance may be futile.
In the midst of deathly calm, a new threat arises, one that will finish the job started by the Cleansing. Tom and Ceri face a stark choice: gamble their lives to try to stop activation of the Beacon, or lie low and wait for the human race to be exterminated. Either way, the end seems certain.