I was thinking something along these lines re-listening to
Season 1 recently. Most of the people in the Tower before it all started were out at work/school/errands/whatever but some folks were at home, I'm sure. So either, there was some initial thing that happened that caused spontaneous "turning" of people throughout LA (probably not given the heat map Micahel & Co produced to ID
Ground Zero) or some outside Zeds came into the Tower (for food, etc) and killed/turned the population that were there.
That doesn't disprove an airborne vector but it probably restricts it.
And as to the number of Zeds vs the number of dead, I'm not sure we have any info on why someone gets turned instead of getting eaten in the course of Zed vs human interaction.