<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by factor XIII deficiency ? [/INST] Inherited factor XIII deficiency affects 1 to 3 per million people worldwide. Researchers suspect that mild factor XIII deficiency, including the acquired form of the disorder, is underdiagnosed because many affected people never have a major episode of abnormal bleeding that would lead to a diagnosis. </s>