There’s something to be said for a book that takes the bewildering cacophony of American approaches to the pandemic and tries to bring some clarity to how we got here. In “American Contagions,” John Fabian Witt, a legal historian at Yale, offers a brisk guide to how epidemics have shaped U.S. law, and how the law has shaped epidemics in turn, taking us from the smallpox outbreaks of the colonies to Covid-19.