There are many species of coccidia that can infect fowl, domestic animals and humans. Each species of coccidia is host specific and does not infect a wide variety of animals. After an outbreak of a specific species of coccidia, the flock will develop a resistance to the exposed coccidia species but remain resistant to other infective species. This means that a flock may experience several outbreaks of coccidiosis, each being caused by a different species of coccidia. Chickens are susceptible to any of nine coccidia species, turkeys are susceptible to seven species and quail are susceptible to at least four different species of coccidia.
Coccidiosis is transmitted by direct or indirect contact with droppings of infected birds.
When a bird ingests coccidia, the organisms invade the lining of the intestine and produce tissue damage as the undergo reproduction. Within a week after infection, the coccidia shed immature descendants that are referred to as oocysts. This sporulation occurs within a one to three day period if the litter is warm and damp but can take much longer if the conditions are cool and dry. After sporulation the coccidia are infective if consumed by a new host bird. The number of infective coccidia consumed by the host is a primary factor as to the severity of the resulting infection.