these were the first bids we've owned in years(we had a chicken but did not own the shed at that point) so there is no "other bird contamination".
I'm guessing it's a safe bet to assume you have some wild birds in your area that would be considered 'other bird contamination"? For example a little sparrow that visits another local chicken keeper and steps in a poop why stealing some chicken feed then lands in your yard, or a duck or goose flying overhead? The oocysts (kinda similar to eggs) are small and can even be transmitted by insects or carried in the air by the wind, and can survive for almost 2 years in that stage... It's best to assume it's everywhere unless you have a highly controlled and isolated sterile environment...
how is this coccidiosis spread, so i can prevent it in my new keets?
It's spread via poop, but essentially you can assume it's everywhere, as wild birds have a tendency to just poop anywhere and don't obey property lines or fences, as said above the oocysts in the poop are small and easily carried and spread... In the end you don't prevent it you control it and allow you keets to build up an resistance so their body can keep it at bay naturally, this is what the medicated feed does, it allows the keets/chicks to get a mild dose of the cocci when exposed so that their body builds up an immunity... There is a lower risk if you keep your coop and run very clean, but you will never eliminate the risk as it's essentially everywhere in the environment... Consider it like any human germ, living in a clean environment, washing your hands regularly, bathing and what not all lower your risk of getting sick but they don't eliminate the risk as germs are everywhere...
also i read that a good way to sanitize the shed is a 10 part water to 1 part bleach mix is this true?
Sure, but that isn't going to make it go away... If you have a contamination problem that would knock down the risk but not eliminate it...
If it is cocci the birds should be medicated with Amprolium so that their bodies have time to build up a natural immunity, then as long as you practice good husbandry in the coop and run the birds should be able to fend off cocci on their own...