Years and years ago, SD required rural water people to send in a water sample when brining home a new baby. Our water was contaminated, which of course scared the wits out of me. I was nursing the new baby so he was safe.
WHAT DO I DO? Oh just open the well and put a little chlorox in there, the directions are on the bottle. Well it takes just a little, and if a little is good....well it seemed like a big well. Yeah, well I had the whitest sinks, toilets, tubs and diapers in the county. We had to just connect the hose and run it out over the bank.
Since then I am much more careful to read the directions and follow them. However, while I try and keep things tidy, I have never washed a coop. Most of the germs will go out of the door with a good brooming.
I have brought home my first coop, and nothing had been in it for years. While I do believe in clean water, and fresh food, I have seen my chickens drink out of mud puddles and eat stuff...well enough said, to each his own way of doing things.
If birds were in this coop recently, well maybe, but most people getting a coop like that is because no one is using the coop. Disease and parasites tend to die out if there is not fresh chicken to feed on.
To each his own, but do read the directions on the bottle, a little goes a long, long way.
Mrs K