Coccidiosis
Coccidiosis is a parasitic infection in which parasites frequently found in soil or bird feces infect the intestines of domestic birds, growing quickly. It can be fatal but any chicken surviving an infection will be immune to future infections. The
video here may be difficult to watch, but is educational.
Medicated feed grower feeds medicated with Amprolium do not prevent infection. Chicks receive low doses of medication to better manage infection once chicks are exposed. This gives chicks a better chance of surviving their initial infection and earning their natural immunity. Feed medicated feed until two weeks after chicks are put outside where they may be exposed.
Corid larger dose of Amprolium used to manage infection. It is easier on the chickens system than other treatments, but does not treat as many strains.
Sumlet 12.5% Solution this is Sulfamethazine, which is harder on the chicken, but effectively treats a wider range of coccidiosis infections.
If you are unable to find either of these locally,
here is a pdf list of medications that may be used to treat Coccidiosis.
Since you know these parasites exist on your property, always have Corid or Sumlet available any time you add chickens to your flock and feed juveniles (up to laying age) feed medicated with Amprolium.