As Bigzio says, anyone can have an infestation occur no matter the precautions taken. Someone could be visiting you and simply track them into your coop or run via their shoes. It might be even that some sorts of pests can be delivered into a chicken run via the poop from a wild bird. That would be a digestive tract type of parasite though, but it is possible. Big hatcheries practice bio-security measures and one is to forbid any but designated employees to have any contact with the birds and then only after passing through a facility designed to eradicate/eliminate any hitchhikers and putting on booties over their shoes.
Best to never allow anyone outside of yourself inside the coop and run. Kids love to see chickens so that can be a hard thing to do. We have allowed others into our run in the past, but we won't do that anymore. It isn't worth the chance, and I only have a flock of 10. (Half of those are on Social Security now at age 5 1/2 yrs.
)
And as others have said on this forum, always expect to do a follow up from 7 -10 days later just to get all of the eggs that hatch. Same procedure as used to treat an animal or human for worms.
BTW, when we built the house we now live in, I thoroughly dusted beneath all vanities and cabinets before installing them. I did all of the baseboard trim that way too. Reason was to kill any future critters which would try to set up shop in those places, whether roaches, ants, etc. 10% Sevin dust kills mice too. It irritates their feet and they try to lick it off resulting in their death. I found one once under our dishwasher. Looked like a taxidermist had done their best on him. He was standing upright with his eyes open. Wish I'd take a picture!
Seriously though, in 26 years here we have yet to see a single cockroach in this house.
Have read on this forum of others who raise roaches in their house in Roach hotels just to feed to their chickens.
Definitely not something I'd want to ever try! Hell, chicken feed isn't expensive enough for me to resort to
that!