Coccidiosis is a frequently diagnosed, but often misunderstood, parasitic infection in sheep. Most sheep are infected with coccidia during their lives but in the majority of animals the parasites cause little or no damage. Disease only occurs if animals are subjected to heavy infections, or if their resistance is lowered. It is, therefore, important to differentiate between infection and disease, as the presence of coccidia does not invariably lead to the development of clinical signs of disease. Indeed, trials have demonstrated that low levels of challenge can be beneficial by stimulating protective immune responses in the host.