The article examines the relationship between a trait’s effect on survival and reproduction and the notion of functional efficiency underlying the biostatistical theory of health (BST). BST faces the problem of how to measure a trait’s joint effect on survival and reproduction in its account of function. If one measures the joint effect by means of the biological notion of fitness, examples such as the hereditary breast and ovarian cancer syndrome do not count as a disorder. If one does not invoke biological fitness, it is unclear how to measure the joint effect while keeping to BST’s naturalist credentials.