Endometritis is an invisible disease. It affects the delicate lining of a mare’s uterus—the endometrium—which can become inflamed and create a hostile environment for sperm, as well as any resulting embryo, to live. Thus, often the only clinical sign of endometritis is not what you do see but what you don’t see: a pregnancy. Endometritis is a major cause of female infertility, affecting up to 15% of broodmares. But because it frequently lacks clear clinical signs, it often goes undiagnosed.