Pet food and even flour can become contaminated with mites. Common house dust mites, of which there are several species, live on human skin scales and mold, though I understand that they can be raised in the lab on cat food. They inhabit dust, wherever it may be. Hot, dry conditions are hostile to mites, whereas warm, damp environments are more conducive to colonization. Basement carpets, if in humid areas, c an also become very infested with dust mites.
Mites travel around within houses and between houses very easily.
I sometimes start to cough and gag when contractors arrive wearing contaminated work clothing. If you sit on an infested furniture cushion, some mites may cling to your clothing. The next place you sit, a few may drop off to start a new colony. In homes with very infested carpets or bedding, the activity of children jumping and playing is enough to make the mites airborne. By looking at dust samples with a microscope, I have found that mites are very fond of grazing in mold on bathroom ceilings and basement walls and even on damp refrigerators cases.