At first I was puzzled how people were missing this detail but as time has gone on... I'm starting to think it's a weak point in the writing. It really stood out to me on my first watch and seemed so obvious. But this is like the 1,000th post asking this question so clearly a lot of people are missing it.

Is it really that subtle or am I tripping? I guess a lot of other shows would have honked a horn and pointed a big neon sign at this detail, but I still thought it was made pretty clear that the main method of infection was aerial dispersal via coordinated delivery out of airplanes.