That is the crux of the issue. I cannot think of a natural phenomena that happens in so many places at once. Mother Nature is more random and does not communicate over global distances in very short periods of time. That brings up intent. If there is intent, there is a conspiracy. Topside people? They are the only ones we know about, but why would anyone pull the trigger on this? Killing 6.5 billion people has gotta be hard on the karma.

An under-Earth intelligence? They get pissed off at us stealing all their dinosaur Kimche that they have stored away to eat later ( oil ) but, now we are fracking, we are not just stealing their food, we are flooding their tunnels with crap. So they attack. I do not think the story will play out his way. I am just stating that I have been looking at so many possibilities.

This also does not jive with ones that are littered with scars and bruises that Michael sees initially. If they contribute to the outbreak, we are then back to conspiracy, because people have to turn them lose in a number of cities.

If it is a gas, why would a scratch from a little one turn Amy in the attack of the colony? We can put something in the blood and have the little ones scratch themselves to get blood underneath their finger nails. But a gas? It could be a virus. Many virus and small bacteria can be next to impossible to see under a standard light microscope unless very strict staining protocols are used.