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. An infection may be mild enough to go unnoticed while a large infective dose of coccidia may produce severe lesions that can cause death. Coccidia survive for long periods outside the bird's body. They are easily transmitted from one house to another on contaminated boots, clothing, free-flying birds, equipment, feed sacks, insects and rodents.
Coccidiosis usually occurs in growing birds and young adults.
It is seldom seen in birds under three weeks or in mature birds. Signs of an outbreak include birds that are pale, droopy, tend to huddle, consume less feed and water, have diarrhea, and may become emaciated and dehydrated. Laying hens will experience a reduction in rate of egg production. Cecal coccidiosis may produce bloody droppings and anemia that is often followed by death. Intestinal coccidiosis is not as acute and is more chronic in nature.