I was recently asked why chickens get coccidiosis, so thanks Danielle for your question.
Chickens get coccidiosis when they ingest the oocysts (a capsule with a thick wall protecting the parasite) that are in the droppings of infected birds. As chickens pick food from the ground they can often pick at the droppings in the litter. Oocysts can also be spread by wild birds, shoes, dust and insects, so it is not just the conditions that the chickens live in that causes the coccidia to be present.
Coccidiosis usually affects younger birds because older birds build up an immunity to the disease once they have been exposed to it. Older chickens can be affected if they have not been exposed to coccidiosis.
I have been told that giving milk to birds that have early signs of coccidiosis will help the birds http://www.cornerstone-farm.com/dealing_with_coccidiosis.htm but the birds can be treated with medication from a vet.