There's a very specific book that I would like to re-read, it's been almost 5 years since I've read it and I cant find it on google. It's about 2 kids, who are faced with a world-ending scenario. The earth creates macrophages to eat humans, and transports them using a mist of sorts. Near the end they discover a signal from the future, and end up sending their own signal back in time. I'm sorry I cant be more specific, that's all I remember.
Could this be The Tomorrow Code by Brian Falkner?
This review mentions a number of the themes you've discussed.
Take a look at a few of the major sci-fi themes discussed: analyzing data from background gamma ray radiation detectors for messages sent backwards in time, macrophages, gene splices spiraling out of control, personal submarines, genetic codes for our destruction living in our own genes, time loops and causality, and more. In fact, if the protagonists hadn’t been kids, I’d argue this was a pretty ambitious novel all by itself.