Rodents will get in!!
Rodents will do their best to gain access into a structure and once in they will continue to nest and quite possible raise their young. As you will notice from the damage to this structure, the rodents have brought in food (seeds) and this is mixed with fecal material and other nasty things. The trouble is not only the animals but parasites and all the nasty urine and fecal materials that are being left behind. Notice that everyone was wearing breathing masks to prevent the possibility of breathing in any fungal spores or contaminated urine or feces dust.