Coccidia are everywhere, you cannot eradicate it from the environment. It most commonly affects young birds and chicks that have not built any resistance. It can affect older birds if they are immune compromised or if they are exposed to a new strain that they have never been exposed to, like when moved to new ground or if new birds are brought in that may carry a different strain. It can also be brought in on clothing, shoes, or by wild birds. Keeping things as dry as possible, and keeping droppings from building up, keeping feeders and waterers clean and dropping free, can help prevent spread. A warm, rainy period where the ground stays wet can cause a 'coccidia bloom' where numbers in the environment are more available for the chickens to pick up while scratching and pecking in the soil. When the birds are exposed gradually to a strain, they build resistance. That is how the medicated feed for chicks is supposed to work, it keeps the numbers down in the digestive tract (it has a very low dose of the same medication in Corid) so the chick doesn't become ill and can build some immunity. Chicks raised by a broody hen are exposed to the microbes in the environment from the beginning and often build resistance naturally. I give brooder raised chicks a dish of dirt to dig around and dust bathe in from day one, in the brooder, to do the same thing. I replace it with clean soil every day. They get exposed to the microbes in the soil the same way, giving them the opportunity to build resistance naturally. I always have Corid on hand in the event anyone does get sick and needs to be treated. Once they show signs of illness then medicated feed is not a high enough dose and you have to treat with full strength Corid. Since I started providing dirt in my brooder I've not had an outbreak in many years. If you get a fecal done then the actual coccidia count is important. A healthy bird can have coccidia in it's digestive tract and be fine, it's only when the numbers get too high that the bird will become sick.
Corid is pretty safe, so if in doubt, I would treat. It won't do any harm. A fecal would also tell you if any other parasites are present, which you would need to treat with a different wormer. If you have a vet that will do that for you, that is where I would start it's not usually terribly expensive. There are also mail in options, but a bit slower to get a result:
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Birds do have cecal droppings several times a day, and they can look a lot different than a regular dropping and still be normal. The lack of appetite and sluggishness would be the thing that suggests more is going on.
