Navajo people frequently attribute occurrences of arthritis and rheumatism to inappropriate contact with menstruating women or menstrual blood. During ethnographic interviews about rules governing contact with various types of blood, Navajo consultants often explained these norms with allusions to key portions of the Navajo oral histories. The connections made by Navajo consultants in these contexts suggest that, like many other diseases, afflictions such as arthritis and rheumatism are metaphorically linked to ancestral impropriety or immorality. That is, particular actions on the part of ancestors of the Nihookáá Dine'é (Earth Surface People) are referenced as the precedent for considering certain types of menstrual and game animal blood dangerous to the health and well-being of contemporary Navajo people. in exploring the means by which these types of blood have come to carry such significance in the Navajo world, I contribute to disciplinary concerns about more effective ways to study so-called menstrual taboos and demonstrate how language, bodily substances, bodily ills, human agency, and ancestral actions intertwine. [Native Americans, Navajo, body, illness, menstruation, sexuality, arthritis/ rheumatism]