The principle articulated in Mishnah Yadayim 3:5 that 'all holy scriptures defile the hands' ((sic)) is one of the most perplexing in rabbinic literature, for how can something that is holy impart uncleanness to the hands? The enigma lies within the principle itself: it is precisely because scriptures are holy that they impart uncleanness; writings that are not holy, such as the books of Homer (m.Yad. 4:6), the Gospels, other heretical books, and the book of Ben Sira (t.Yad. 2:13), do not impart uncleanness. What I should like to propose in this article is an explanation that seeks to clarify the principle of tum'at yadayim by the concept of holiness as a sacred contagion. Holy scriptures, like other holy objects of cultic worship, were considered sources of contamination. But unlike them, holy scriptures do not render objects with which they come into contact holy; rather they make the hands unclean. Holy scriptures were considered a particular kind of sacred contagion. It is suggested that the key to understanding the defilement principle is found in the traditions about the Ark of the Covenant and its lethal power to kill those who are ineligible to touch it.