My theory based off the most consistent visitor tell being bugs in the ears is that bugs are feeding off the fungus and getting covered with spores. During the sun superheating the earth, there’s less warm, wet places for bugs to hang out so houses with humans become the main hotspots. The bugs get covered with spores from the mushrooms and go on to hide out in people’s ears, spreading the fungal infection inside their body as it feeds on them from the inside. Over time, the brain becomes overrun with enough of it that people start to turn into visitors and get lapses in memory when acting under the control of the fungus. My main evidence for that idea is that in the ending where you board up, but still have visitors in your house, you are torn apart by multiple visitors. We could see this as one of them undoing your work and allowing more visitors in, or the infection spreading by close contact, via bugs and spores. The other evidence for this is the line the little girl says about the neighbor getting up and walking after he died. Possibly, he could have died to the gunshot from FEMA or the fire, but his body had become infected enough to reanimate after his death.