Severity of coccidiosis depends on the number of the organisms in the intestinal tract. If only a few are ingested, no symptoms may occur. Repeated mild exposure may produce immunity to the organism without disease. If a moderate number are ingested, the disease may be mild and immunity may result. If, however, poor sanitation, crowding, or stress increases the number of organisms ingested or the ability of the organisms to multiply rapidly, severe disease and even death can result.
Coccidiosis is most common in young animals because they have limited immunity to the organism.
Adult animals which remain in the herd are usually immune to the local coccidia. Bringing in new animals can cause an outbreak of coccidiosis in the new additions, or the new animals may bring in a new species of coccidia and cause an outbreak in the original herd. The immunity to coccidiosis is not well understood. Immunity is species specific. For example, animals which are immune to one species of coccida organisms are still susceptible to the 15 other species.