Corid is the brand name of the amprolium medication commonly used to treat coccidiosis.
How old are the birds in your flock and how long have you had them?
Coccidiosis and the protozoa that causes it are everywhere in the soil. Most birds that are exposed over time build resistance to it and will only become ill if the numbers of coccidia become too great, or the bird is weakened with another illness. Young chicks are most at risk as they have not built any resistance yet. There are 9 strains that effect chickens so they build resistance to those they are exposed to. Moving to a new area or bringing new birds in can expose them to new strains that they are not resistant to, or carrying new strains in on your shoes from another area is possible.
The common signs of coccidiosis are lethargy, loss of appetite, sitting hunched/fluffed up, and runny/mucousy/sometimes bloody droppings. If you are not seeing any signs of illness then there is no reason to treat. Having Corid on hand so you can begin treatment quickly in the event one does get sick, especially if you have chicks, is a good idea. Many people keep Corid on hand as part of their first aid/health supplies. Medicated feed is used by some as a preventative for young chicks, but it will not treat an actual outbreak, for that you will need to treat with Corid.
Keeping housing and bedding clean and dry (dry is important) and minimizing droppings build up in all areas they are in, and keeping feeders and waterers clean and free of droppings are the best ways to help prevent outbreaks. If there are areas where they range/live that stay wet for periods of time and don't drain well, those can contribute to outbreaks.
Here is a link to more info:
https://www.backyardchickens.com/articles/coccidiosis-how-to-treat-it.64386/
Hope this helps.