Coccidiosis usually occurs only if an animal is subjected to heavy infection when its resistance is low. Low resistance can be related to age, immune status, nutrition, or stress. Signs of intestinal coccidiosis usually include diarrhea, with or without blood loss. Lesions are caused primarily by inflammation, epithelial hyperplasia, and epithelial loss. They include villus atrophy, mucosal thickening, polyps, crypt atrophy, denudation, and hemorrhage. The most pathogenic species tend to be those that destroy crypt stem cells in the large intestine, causing denudation and hemorrhage.